Transformer Questions

[tvr2500m]

I'm putting together some kits. A headphone amp and a small integrated amp (15wpc). I've got boards and components to stuff the boards. Each has onboard power rectification and filtering. I have to source transformer/s, volume pots/controls/attenuators, input and output jacks, switches (where desired/appropriate), and enclosures/casework.

I'm good with stuffing boards and soldering. No problem here. I do have a question about what the variable resistor/s is used for and what to do with it in the integrated amp, but I'll get to that when I, well, get to that.

I am sorting out what to do with the line power input. I need some transformers to step down wall power. The schematic of the little integrated indicates 9V and 15V inputs to the power supply, and the headphone amp shows it's looking for just 15V in.

My questions are:

1. What type of transformer "should" I use. Torroids? EIs/regular laminated? R-cores?
2. What size do I need and how is this measured? I see VA and A specs, among all the other specs that surround transformers.
3. What's the best way to include some line protection? I've seen IEC jacks with built-in fuses. If I do a regular line fuse block, exactly how do I wire this up?

Parts Express has Avel torroids. They have a nice-looking 15V, but don't list a 9V in the online catalog.

Thanks in advance for any useful insights.

- SJ

[WWWJD]

Sounds to me like you need a 15VAC Wall wart, if the rectifiers and filters are already in place; T-Amps? It almost sounds like you're trying to kick off some sort of mass production here, with very little experience, no offense.

I'm sure there are plenty of people around here that can explain away all of your questions; but we charge no less than $100/hr... (j/k)

Not to steal PE's thunder, but there's a pretty good forum on 41Hz.com which by first blush, is more suited to what you're doing.

[Æ]

I'm putting together some kits. A headphone amp and a small integrated amp (15wpc). I've got boards and components to stuff the boards. Each has onboard power rectification and filtering. I have to source transformer/s, volume pots/controls/attenuators, input and output jacks, switches (where desired/appropriate), and enclosures/casework.

I'm good with stuffing boards and soldering. No problem here. I do have a question about what the variable resistor/s is used for and what to do with it in the integrated amp, but I'll get to that when I, well, get to that.

I am sorting out what to do with the line power input. I need some transformers to step down wall power. The schematic of the little integrated indicates 9V and 15V inputs to the power supply, and the headphone amp shows it's looking for just 15V in.

My questions are:

1. What type of transformer "should" I use. Torroids? EIs/regular laminated? R-cores?
2. What size do I need and how is this measured? I see VA and A specs, among all the other specs that surround transformers.
3. What's the best way to include some line protection? I've seen IEC jacks with built-in fuses. If I do a regular line fuse block, exactly how do I wire this up?

Parts Express has Avel torroids. They have a nice-looking 15V, but don't list a 9V in the online catalog.

Thanks in advance for any useful insights.

- SJ

Be aware that transformers are AC voltage. If you need DC voltage, you have to rectify and filter the AC voltage. Rectifying an AC voltage into a DC voltage, changes the voltage quite a bit too.

[dthomas]

I'm putting together some kits. A headphone amp and a small integrated amp (15wpc). I've got boards and components to stuff the boards. Each has onboard power rectification and filtering. I have to source transformer/s, volume pots/controls/attenuators, input and output jacks, switches (where desired/appropriate), and enclosures/casework.

I'm good with stuffing boards and soldering. No problem here. I do have a question about what the variable resistor/s is used for and what to do with it in the integrated amp, but I'll get to that when I, well, get to that.

I am sorting out what to do with the line power input. I need some transformers to step down wall power. The schematic of the little integrated indicates 9V and 15V inputs to the power supply, and the headphone amp shows it's looking for just 15V in.

My questions are:

1. What type of transformer "should" I use. Torroids? EIs/regular laminated? R-cores?
2. What size do I need and how is this measured? I see VA and A specs, among all the other specs that surround transformers.
3. What's the best way to include some line protection? I've seen IEC jacks with built-in fuses. If I do a regular line fuse block, exactly how do I wire this up?

Parts Express has Avel torroids. They have a nice-looking 15V, but don't list a 9V in the online catalog.

Thanks in advance for any useful insights.

- SJ

Since you have on board filtering and rectification which type of transformer is not really important. E/I can vibrate but that is usually not an issue, toroids are usually lower profile wich can be a benefit.

I have bought several transformers from these guys. Are you sure your integrated amp does not want 9V to 15V odd they would need two AC supply voltages, I would use a 10VA 9V for the headphone amp and a 50VA 15V for the amp. The 50VA is a bit overkill for a 15W amp but having plenty of grunt in your power supply is a good thing.

If you are unsure post a link to the schematic of your integrated amp kit.

http://www.antekinc.com/trans.html

[tvr2500m]

Thanks for the replies. Dave's got it. I just have to source some pieces to complete the parts list for these kits I've bought. Yeah, these kits aren't beasts, so maybe even a wallwart would work, but I'm thinkin' that something just a little more substantial might be more interesting and fun. "Sound better"? Dunno, but... Yes, looking at similar gear I've got from what I feel to be well-regarded audio products companies, the transformers aren't anywhere near 50VA, so this is overkill here, too. But, some of the gear do/did offer optional transformer "supplies" as "upgrades" from a wallwart.

I've attached schematics of the headphone amp and integrated amp, and pictures of each. The Virgo headphone amp picture show what I'm going after for this piece.

Thanks!

- SJ

[dthomas]

Thanks for the replies. Dave's got it. I just have to source some pieces to complete the parts list for these kits I've bought. Yeah, these kits aren't beasts, so maybe even a wallwart would work, but I'm thinkin' that something just a little more substantial might be more interesting and fun. "Sound" better? Dunno, but... Yes, looking at similar gear I've got from what I feel to be well-regarded audio products companies, the transformers aren't anywhere near 50VA. But, they do/did have optional transformer supplies bits offered as "upgrades" from a wallwart.

I've attached schematics of the headphone amp and integrated amp.

Thanks!

- SJ

The Antek transformers are very reasonable. Acutally on the 15V it supplies the OP2134 opamps so 5 or 10VA is sufficient. The 9V is for the amplifier power stage I would go at least 20VA. That is nice little kit with excellent opamps and output transistors. But its dynamic range will only be as good as the power transformer that feeds it. If you supply it too little power from a wimpy wall wart you will net a wimpy output.

It appears the integrated amp has two options for supplying the opamps one regulated with the LM317/337 and the other from the 9V supply. Either will work fine though the opamps might be a bit quiter on a regulated supply vs. the unregulated. The opamp will be happy with either the 9V or the regulated 13.75V supply from the regulator circuit. Which are you using? The headphone amp will be fine with 5 to 10VA.

[tvr2500m]

Thanks, Dave. This REALLY helps my understanding. Okay, so the integrated amp kit gives a choice of 9V or 15V inputs. I didn't understand why the schematic shows these two voltage inputs, now I do. The 9V is unregulated, the 15V outputs a regulated 13.75VDC output.

I have a good idea of the capacity of the transformers. Again, thanks.

Not having known the voltage input choice, I hadn't made a choice about which to use. I'm thinking the 15V input regulated, per your suggestion.

I think I'm all set for sourcing all the bits for the "Virgo" headphone amp. And I'm much further along understanding how to proceed with the little Q10 integrated amp.

If I may ask a few more questions:

1. Why the two choices of voltage inputs on the Q10 integrated? Why might this have been designed this way? Advantages/disadvantages to either approach?

2. The Q10 has a variable resistor, or two (don't recall exactly at the moment). What do these adjust? The bias of the op amps? How do I set this/at what points to do take bias measurements? And, what is the bias range? I do have documentation on bias adjustment for the Virgo, so I'm all set there.

3. What attenuator/potentiometer resistance value should be used with the Q10? 10K, 50K, 100K, 250K? I was considering an Alps "Blue Velvet" or inexpensive stepped discrete resistor ladder thing. I just need to know what resistance value should be used.

4. Could the Virgo headphone amp also be used as a preamp? Is this as easy as just providing another set of output jacks?

Thanks for all of the help. I really appreciate it.

- SJ

[dthomas]

If I may ask a few more questions:

1. Why the two choices of voltage inputs on the Q10 integrated? Why might this have been designed this way? Advantages/disadvantages to either approach?

:)Well you have to use the 9V supply that is where V2 pos/neg rails come from. The supply V1 can be taken from the regulators or from the 9V supply board. Odd they did it this way expect it gives you the option of using a single AC transformer. The only advantage to the regulated supply is the opamps would be slightly quieter on a regulated supply. It is doubtful that the LM317/337 would supply sufficient current for the output stages plus the extra voltage may make the amp unstable especially into a low impedance speaker load so don't use the regulated supply for V1 rails. You might want to ask the manufacturer why they did this only logical reason I can see is that is a way to use a single transformer if you want to.

2. The Q10 has a variable resistor, or two (don't recall exactly at the moment). What do these adjust? The bias of the op amps? How do I set this/at what points to do take bias measurements? And, what is the bias range? I do have documentation on bias adjustment for the Virgo, so I'm all set there.

:(I don't have the schematic with me but maybe for DC offset hard to say. Maybe someone else can see it. The pic is kind of small on a blackberry.

3. What attenuator/potentiometer resistance value should be used with the Q10? 10K, 50K, 100K, 250K? I was considering an Alps "Blue Velvet" or inexpensive stepped discrete resistor ladder thing. I just need to know what resistance value should be used.

:)Use a 50K or 100K. I usually find 100K works best for most applications.

4. Could the Virgo headphone amp also be used as a preamp? Is this as easy as just providing another set of output jacks?

:)Sure it can typically a headphone amp just has lower output impedance compared to a typical preamp. Gain might be a bit high but assuming you don't crank the volume too high then that is not a real issue. In short it will not hurt the headphone amp to drive the input of an amp your issues will only be how much output voltage it has so if it starts to distort the amps output turn it down. Oh you want to add an output capacitor to the preamp output jacks you add just to make sure you block any dc from the OPA2134's inputs. 1.5uf to 2uf is a good value just use a good quality poly cap. This will make sure you don't cause the opamps in the front end of the integrated amp to fail. But the opamps in the integrated amp are acting as a preamp so why have a seperate preamp just add a volume control?

[lousypilot]

Toroidal transformers are generally preferred as they are self-shielding and therefore won't induce noise into adjascent circuitry (or very little comparatively). They are also somewhat expensive!

The VA (volt/ampere) rating is called "Apparent Power" (Pa), the unit of measure is Watts. As transformers change and pass power (most of it anyway), rather than dissipate the power, is where the distinction lies. So transformers are rated in VA rather than Watts. The voltage applied to a load multiplied times the current flowing gives power in Watts. In transformer terms, it is the power being transferred to the load device.

The Avel 30VA toroidal transformer you mentioned is a 1 amp transformer, meaning it can supply 1A of current to the two 15V secondaries. 30VA=(15v+15v) x 1A. The first 250VA x-fmr (122-620) is nearly 7A. 250VA=(18v+18v) x 6.9A. So to choose a suitable transformer, you need to know the maximum power draw of the load device. It must not exceed the VA rating of the transformer.

If the amplifier you're building does not need a "split" supply (+15v and -15v), a wall adaptor for your power supply might be the way to go. To go along with the power transformer, you'll need filter capacitors, a bridge rectifier, and possibly even voltage regulation, to build a suitable power supply. This could be a tall order for you without some electronics training.

I hope this helps!
Eric @ PE

[tvr2500m]

Thanks for sticking with me on this. I would love to be able to contact the manufacturer. This would all be easy if that were an option. I bought these kits from an eBay'er who disappeared. You can find reports of customer issues around the internet. I think these designs originated at the DIYZone in China. There's stuff on the boards and on that Q10 schematic pointing to this. But, while I've found enough information about the Virgo headphone amp to build it up, I haven't been able to turn up anything searching for information on this little integrated amp. Interpretation services provided by the search engines are certainly helpful, and much better than nothing, but still leave much to be interpreted ;)

Okay, so I have to use the 9V inputs, too? The board has terminal blocks for both the 9V and 15V right next to one another. I've seen listed for use with this little amp a "T96" toroidal transformer (which I have specs on and I've also seen spec'ed for use with the Virgo) that has both 9V AND 15V outputs. I've attempted to contact a DIY audio vendor in China that lists this transformer, but I haven't received a reply yet. Shipping from China is pricey, and unfortunately I haven't been able to find a similar transformer from any other source.

I don't have a problem running two transformers. A couple blue potted PCB toroids would be nifty. I'd just connect them from the IEC jack, then to the respective terminal blocks and I'm good to go? That's easy, if that's what has to be done. Or is some other type of connection required.

The plan for the two pieces is to give me a headphone amp to play with and nice, small amplifier for 2-channel TV/HT duty. I've ended up with nice Sennheiser 580 headphones, but no decent amp, and no way to use the headphones with anything more exciting than an iPod or computer (both of which can sound just fine). I've borrowed a few good headphone amps, but this is the first one I've owned. Some of these headphone amps also serve as preamps, so I thought, just out of interest, to set this up as a preamp, too, and see how it sounds/works.

With the little Q10 integrated amp, I'm looking for a simple, small, good enough-sounding amplifier for 2-channel TV/HT use, driven by a TV output. I've alternately had an Adcom 5300 and a vintage Yamaha CR-820 receiver doing this job (driving some Polk RT25is along with a Polk PSW100 sub), but I'd like downsize/simply. I'll likely hook it up to more serious gear just to see how it does. I hadn't/don't plan to use the Virgo headphone amp as a preamp for the Q10.

So, is selecting an attenuator's resistance that easy? Just grab a 50K or 100K and call it a day? What's the effect of the resistance/role in the circuit? I have some "documentation" on the Virgo that suggests that a "complete" kit was fitted with a 10K pot, so I was planning to use one of these, but could certainly just as easily use something else.

Of course, applicable to all of this, is to learn something about electronics. I've built LOADS of kits. Gimme all the bits and proper instructions and I'm good to go. The absence of all the bits and assembly instruction are helping/forcing me learn at least just a little more about electronics.

Thanks again!

- SJ

[dthomas]

Thanks for sticking with me on this. I would love to be able to contact the manufacturer. This would all be easy if that were an option. I bought these kits from an eBay'er who disappeared. You can find reports of customer issues around the internet. I think these designs originated at the DIYZone in China. There's stuff on the boards and on that Q10 schematic pointing to this. But, while I've found enough information about the Virgo headphone amp to build it up, I haven't been able to turn up anything searching for information on this little integrated amp. Interpretation services provided by the search engines are certainly helpful, and much better than nothing, but still leave much to be interpreted ;)

Okay, so I have to use the 9V inputs, too? The board has terminal blocks for both the 9V and 15V right next to one another. I've seen listed for use with this little amp a "T96" toroidal transformer (which I have specs on and I've also seen spec'ed for use with the Virgo) that has both 9V AND 15V outputs. I've attempted to contact a DIY audio vendor in China that lists this transformer, but I haven't received a reply yet. Shipping from China is pricey, and unfortunately I haven't been able to find a similar transformer from any other source.

I don't have a problem running two transformers. A couple blue potted PCB toroids would be nifty. I'd just connect them from the IEC jack, then to the respective terminal blocks and I'm good to go? That's easy, if that's what has to be done. Or is some other type of connection required.

The plan for the two pieces is to give me a headphone amp to play with. I've ended up with nice Sennheiser 580 headphones, but no decent amp, and no way to use the headphones with any more exciting than an iPod or computer (both of which can sound just fine). I've borrowed a few good headphone amps, but this is the first one I've owned. Some of these headphone amps also serve as preamps, so I thought, just out of interest, to set this up as a preamp, too, and see how it sounds/works.

With the little Q10 integrated amp, I'm looking for a simple, small, good enough-sound amplifier for 2-channel TV/HT use, driven by a TV output. I've alternately had an Adcom 5300 and a vintage Yamaha CR-820 receiver doing this duty (driving some Polk RT25is along with a Polk PSW100 sub), but I'd like downsize/simply. I'll likely hook it up to more serious gear just to see how it does. I don't plan to use the Virgo headphone amp as a preamp for the Q10.

So, is selecting an attenuator's resistance that easy? Just grab a 50K or 100K and call it a day? What's the effect of the resistance/role in the circuit? I have some "documentation" on the Virgo that suggests that a "complete" kit was fitted with a 10K pot, so I was planning to use one of these.

Of course, applicable to all of this, is to learn something about electronics. I've built LOADS of kits. Gimme all the bits and proper instructions and I'm good to go. The absence of all the bits and assembly instruction are helping me learn at least just a little more about electronics.

Thanks again!

- SJ

Selecting the resistance of the volume pot is fairly simple. I like 100K as it gives you a lot of range very gradual changes in volume. The Antek toroids are $10 to $13 each depending on VA rating. I have used several of them and never had an issue. They come with mounting hardware and the guy who runs the company is great ships fast. All you have to do is connect the transformers to the AC coming in and then wire them to your terminal blocks, rectification, filtering etc... already seems to be on your boards at least they are in the schematic. Is the integrated amp on one PCB?

If you are just using it for TV use the 9V and connect V1+ and V1- in that supply to the opamps. I assume there is a jumper or something that determines where V1+ and V1- is fed from. You will have a lot of trouble finding a transformer that has 9 and 15V secondaries. The OPA2134 is happy with as little as 2.5V so you should give it enough voltage to drive the amp fine. May not reach full output power with out 15V rails.

[philiparcario]

antek is very good. see link to my 5 channel gainclone


http://cgi.ebay.com/Gainclone-gain-clone-5-channel-sale-50-off_W0QQitemZ150344432054QQcmdZViewItemQQptZLH_Def aultDomain_0?hash=item230139f9b6&_trksid=p3286.c0.m14#ht_500wt_1182

that is a 400va trans from antek works great. i sold a modded gainclone to a board member here and put in a nice antek trans for him works great.

the blue velvet pots are good.

link for mono pots

www.partsconnexion.com/product7381.html
12 each they sell stereo but if you are doing all the work mono gives far better control and will tracking perfectly 24 a pair stereo are 18 each

[tvr2500m]

If the amplifier you're building does not need a "split" supply (+15v and -15v), a wall adaptor for your power supply might be the way to go. To go along with the power transformer, you'll need filter capacitors, a bridge rectifier, and possibly even voltage regulation, to build a suitable power supply. This could be a tall order for you without some electronics training.

I hope this helps!
Eric @ PE

Thanks, Eric. Yes, this does help. All the supplies seem to be "split" - 15V-0-15V or 2x15V, and 9V-0-9V.

I appreciate the transformer description. I have some macro/fundamental understanding of this, though little practical application (heck, I started college as an EE). Your posts helps me fill in some gaps.

I understand the broad sizing requirement, but the question, and I suppose the answer involves some complexity, that remains is how do you exactly calculate the xformer size so that it isn't a bottleneck to a circuit?

Both the Q10 integrated amp and Virgo headphone amp have full ps bits, they just need to be fed the right voltage.

- SJ

[tvr2500m]

antek is very good. see link to my 5 channel gainclone


http://cgi.ebay.com/Gainclone-gain-clone-5-channel-sale-50-off_W0QQitemZ150344432054QQcmdZViewItemQQptZLH_Def aultDomain_0?hash=item230139f9b6&_trksid=p3286.c0.m14#ht_500wt_1182

that is a 400va trans from antek works great. i sold a modded gainclone to a board member here and put in a nice antek trans for him works great.

the blue velvet pots are good.

link for mono pots

www.partsconnexion.com/product7381.html
12 each they sell stereo but if you are doing all the work mono gives far better control and will tracking perfectly 24 a pair stereo are 18 each

Thanks. Nice gainclone. Okay, so we like Antek toroids. 400VA sounds like a bit more xformer than might be required.

So, you're suggesting a pair of mono potentiometers rather than a single stereo?

- SJ

[tvr2500m]

I found some interesting information on the little Q10 mini integrated amplifier - http://www.diyzone.net/article.php?sid=1119. It appears to be an article by its designer. It's a bit difficult to sort it all out from a Google page translation. One interesting bit about the 9V/15V powersupply from the translation:

"Power supply is divided into two parts, pre-and post-class separate from the overall performance of power supply upgrade, and to impose some pre-regulator to ensure that the overall quality of fine degree."

It looks like the ps has two "stages" to improve ps performance; at least so is the design intent. From the pictures of the Q10 in the article, "T1" and "T2", which I've been wondering what these bits of the circuit are all about as I ponder the schematic, are bridged with jumper. I've attached a picture of amp and block circuit diagram.

- SJ

[philiparcario]

Thanks. Nice gainclone. Okay, so we like Antek toroids. 400VA sounds like a bit more xformer than might be required.

So, you're suggesting a pair of mono potentiometers rather than a single stereo?

- SJ

for you 400va is overkill, but i always suggest mono pots for volume controls. you can have perfect matching levels. stereo pots sometimes track not matched. for 6 more dollars i feel it is worth doing.