Buffered Volume Control

[AJ]

This is a project BrokenCrank and me are working on.

It's a stereo buffered volume control based on an Alps "Blue Velvet" pot and OPA2134 opamps connected as unity gain buffers presenting an easy load to the source no matter where the volume control is positioned, and a low output impedance to drive a power amplifier. There is also a relay on the board which shorts the outputs at turn on/off. This means you can turn the preamp on or off with the amp on and not get any thumps or noises. I used Rod Elliot's (sound.westhost.com) regulated +/-15Vdc power supply board with an Antek Inc. transformer to power the opamps. His board also has the time delayed voltage supply for the relay, which is handy.

I have more of the PCB's if anyone wants one. The one we're building has one stereo input, but one could be built with up to 6 stereo inputs using a simple rotary switch. These could be used to create integrated amps, or built standalone like this one. Basically, this is an improved version of the "passive preamp". It really is just a volume control as there is no voltage gain, but it doesn't have the drawbacks of just using a pot by itself. The interconnects can be any length (within reason) and it can be used with most power amplifiers.

I paid about $8ea to get them manufactured, so I'd like to get $10ea to cover postage anywhere in the country. Some quick pics below, schematic and bill of materials will follow.

[greywarden]

Re: Buffered Volume Control

That's cool, looks relatively inexpensive, too.

[AJ]

Re: Buffered Volume Control

The PCB's are two layer, 0.063" FR4 with soldermask and silkscreen.



Blue is the bottom layer, red is top.



Silkscreen layer...



Schematic...

[Tin_Ears]

Re: Buffered Volume Control

Any plans to make a 4 or 6 or 8 gang version?

[fastbike1]

Re: Buffered Volume Control

Looks like a winner. Waiting for purchase details.

[AJ]

Re: Buffered Volume Control

Any plans to make a 4 or 6 or 8 gang version?

Not right now. More than two channels isn't really practical with off-the-shelf pots. You can get step attenuators with that many gangs, but they're not cheap. PGA2310's are useful for controlling that many channels.

Looks like a winner. Waiting for purchase details.

I'll have the bill of materials done tomorrow.

[Face]

Re: Buffered Volume Control

Any plans to make a 4 or 6 or 8 gang version?

Purchase two boards and use jumpers to a 4 gang pot/attenuator.

[50 watt head]

Re: Buffered Volume Control

I may be interested in one of these boards, as well.

[robertcottiers]

Re: Buffered Volume Control

Hi AJ: Do you have power supply decoupling capacitors from each power supply pin to ground?.. Your schematic shows C2 and C6 connected across the +/- 15 volt supply...Unless i am missing something.

Nice work on the project!

Regards
Bob C.

[AJ]

Re: Buffered Volume Control

Hi AJ: Do you have power supply decoupling capacitors from each power supply pin to ground?.. Your schematic shows C2 and C6 connected across the +/- 15 volt supply...Unless i am missing something.

Nice work on the project!

Regards
Bob C.

C3, C4, C7, C8 go from each rail to ground where the power enters the board. C2, and C6 are right next to the ICs. This seems to work fine for a small board like this. If the ICs were configured for high gain and were far away from the main decoupling caps I'd want local decoupling right next to them.

[AJ]

Re: Buffered Volume Control

Bill of materials is done. This is just to assemble the board itself. You would still need the power supply, case, jacks, etc...

For the power supply I used the ESP P05B

The power transformer I used is an Antek AN-0115

[foxfire3]

Re: Buffered Volume Control

Very nice work AJ!

I'm currently using the old Analog Device's BUF-03 buffers for my buffered volume control...better known as Aunt Corey's Buffered Passive Preamplifier.

[AJ]

Re: Buffered Volume Control

Very nice work AJ!

I'm currently using the old Analog Device's BUF-03 buffers for my buffered volume control...better known as Aunt Corey's Buffered Passive Preamplifier.

Thanks foxfire. What made you choose the BUF-03's?

On the BOM i have the molex connectors and crimp terminals listed as optional as most people probably won't bother with them. I used them because they are convenient, and I have access to the crimper at work. The crimper for the contacts is pretty expensive, and not really worth it to most diyers. The board will take 22-20AWG wire directly.

[revelator]

Re: Buffered Volume Control

Thanks foxfire. What made you choose the BUF-03's?

Regarding the BUF-03, perhaps Foxfires' inclination to build was fueled by the two-part article in Stereophile magazine a few years back. I tried and was not successful, then gave up after reading that Burr-brown OP amps are suppose to be better sonically. In the end, I ended up building a passive attenuator with transformer/inductance (...from company in England, Sowter) instead of using carbon pot for ultimate signal fidelity? Are the OPA2134 you mentioned Burr-Browns?

Wonder how the buffer will work with inductor attenuators....Keep us updated.. great work

[AJ]

Re: Buffered Volume Control

Yes, the OPA2134 was a Burr-Brown until Texas Instruments bought them. I think the new ones I bought still have the BB logo, though so I doubt they've changed any.