Amplifier Gugu needed, Hafler and Parasound setup

[siggma]

Greetings,

I recently purchased a preamp and amplifier for listening. I have a few questions and I'm hoping someone with some amplifier knowledge can help me out. I'm also a speaker builder. I bought the amp and preamp to use for testing as well as listening. The preamp is a Hafler 915 without the Phono option. I haven't purchased a source player yet. I'm looking at possibly getting an SACD player but I haven't done enough research yet. I also intend to use it with my calibrated mic for measurements. I purchased an external phantom power box and cables to hook it up to the Hafler input. I measured a full knob gain of 20 (60.9 mv input, 615 mv output) in the preamp. I hooked it up in a loop back to my sound card and ran some distortion measurements using STEPS (part of the ARTA package). First is Frequency Response 10-50Khz. I didn't use a fully shielded cable on the sound card input and I don't know if it will effect any of the measurements or not. It looks OK but then I don't know exactly what to look for.

Is this "GOOD" in terms of measurement?
If I read this correctly I'm getting very good S/N (greater than measurement capability which means >100db) and relatively low distortion as well. FR is dead flat from 10hz to well above 20K so that's good. But I don't really know how to read distortion measurements.


Here is the distortion alone. It's advertised at .008% but I think that's at 1K only. I'm seeing .7% THD driving it with about .5v. Cursor is at 1K so text below indicates SN ratio or distortion floor? I also notice THD or D3 increases below 50hz. Any ideas what might cause this, alligator clip cables?


I could also set the output up higher and run a linearity function test.

Last question I can't show you because I'm afraid I'll cook another sound card. The amp is a Parasound 2125. It sounds OK to me but I haven't listened to it for any length of time. It's definitely not "hot" like many of the other THX amplifiers I've heard. It may have the THX movie correction (+3db 5K-20K?) applied to FR to correct for movie sound tracks. In any case I tried to measure it using my laptop. With all levels down, input loaded by sound card output I measure .2mv (.0002v) DC at the output and 63mv of noise that I think comes from the computer output. When I touched the sound card input to the amp output it popped and it cooked the sound card OUTPUT, not the input, which is weird. I need to resolve this before I try again.

Anyone know why this might have happened? I'll measure between the amp output and card input when I get another one working but I'd like to get some feedback in the mean time. I definitely hear noise from the computer but it's dead quiet with the input unloaded. I think it has an input shunt to ground when unloaded, it also has an "auto-on" feature so it turns on with other equipment.

Any thoughts would he helpful.

Thanks.

[AJ]

Re: Amplifier Gugu needed, Hafler and Parasound setup

The 915 is a pretty nice preamp. I haven't done any testing with ARTA yet, but the junk in the frequency response measurement above 25Khz shouldn't be there. I would assume that's artifacts from the A-D conversion at the sound card, and not the Hafler. The high distortion could also just be the sound card. A FR measurement of the sound card itself (loop output to input) would be helpful.

In case you didn't already know, the manual for the 915 with schematic is available at Hafler.com

According to the owners manual the Parasound is +0/-3db from 20hz to 50Khz so no THX correction. Did you have a load on the amps output?

[DE Focht]

Re: Amplifier Gugu needed, Hafler and Parasound setup

Greetings,


Last question I can't show you because I'm afraid I'll cook another sound card. The amp is a Parasound 2125. It sounds OK to me but I haven't listened to it for any length of time. It's definitely not "hot" like many of the other THX amplifiers I've heard. It may have the THX movie correction (+3db 5K-20K?) applied to FR to correct for movie sound tracks. In any case I tried to measure it using my laptop. With all levels down, input loaded by sound card output I measure .2mv (.0002v) DC at the output and 63mv of noise that I think comes from the computer output. When I touched the sound card input to the amp output it popped and it cooked the sound card OUTPUT, not the input, which is weird. I need to resolve this before I try again.

Anyone know why this might have happened? I'll measure between the amp output and card input when I get another one working but I'd like to get some feedback in the mean time. I definitely hear noise from the computer but it's dead quiet with the input unloaded. I think it has an input shunt to ground when unloaded, it also has an "auto-on" feature so it turns on with other equipment.

Any thoughts would he helpful.

Thanks.

Just FYI, THX certified amplifiers do not have any frequency compensation built in (seriously?), THX has separate specifications for amplifiers which are quite modest. The THX processor has the correction, but (going from memory) since the day of Laser Discs this has not been a problem. As there is enough money in selling and renting DVD's that most movie soundtracks on the disc are mixed for home use.

BTW THX movie correction would be -3dB, so the amplifier would not be "hot".

[siggma]

Re: Amplifier Gugu needed, Hafler and Parasound setup

The 915 is a pretty nice preamp. I haven't done any testing with ARTA yet, but the junk in the frequency response measurement above 25Khz shouldn't be there. I would assume that's artifacts from the A-D conversion at the sound card, and not the Hafler. The high distortion could also just be the sound card. A FR measurement of the sound card itself (loop output to input) would be helpful.

In case you didn't already know, the manual for the 915 with schematic is available at Hafler.com

According to the owners manual the Parasound is +0/-3db from 20hz to 50Khz so no THX correction. Did you have a load on the amps output?

Thanks for the response. I haven't had a chance to set up the Parasound amp yet. I just bought a new M-Audio Audiophile sound card so I have a decent analog source when I get it set up. I don't want to have my only source be my media PC analog output. I'll re-run the tests with the new card at 192K and see if it's better. I also just realized the sound card was probably only running 48K or 96K. I don't think that's enough nyquist frequency to get to 50K off the top of my head. The new Audiophile card is absolutely dead quiet unlike the old internal card. I can't even hear a pop when I hook up a speaker to the amp, it's that quiet. With the Hafler preamp it should be a very nice listening setup. I haven't figured out how all the gain controls work on the card yet and it has its own ASIO control panel so it might take me a while, I have a lot of new toys to play with. What I'm wondering is what in the amp circuit would drive enough current with no signal applied to cook the laptop sound card. It wasn't a signal, it had to have been some kind of wiring thing. Now that I can run it again I'll see if I can track down the current source that did the cooking.

[Paul O]

Re: Amplifier Gugu needed, Hafler and Parasound setup

When I touched the sound card input to the amp output

Did you state that wrong because normally it would be the sound card output to the amp input.

[siggma]

Re: Amplifier Gugu needed, Hafler and Parasound setup

Did you state that wrong because normally it would be the sound card output to the amp input.

The sound card output is connected to the amp input. And the amp output is connected back to the sound card input. I was setting up to calibrate for an impedance measurement. Normally I use a jig with zener protection on the input but I've done it without the zener protection too and never had this happen, even on my BASH sub amplifier which I can't use because of the permanent internal sub filters. It comes with an ominous warning not to try to connect it to any test equipment, like it has some kind of floating ground or DC feedback loop or something. I've connected it up this way before and it works fine as long as I keep the amp volume low.

I've been using the sound card output to drive the speaker for measurements and it works OK but it's not completely accurate for various reasons, low current being one, so I want to use an amplifier to drive the speaker to get better damping and a more accurate Z max (maximum impedance at speaker resonance). The only other way to "bench" the driver is to do it by hand using an AC voltmeter or oscilloscope but my scope is on the laptop too. I guess I'll need to shrink tube a zener protection circuit and make some probes.

The impedance measurement uses two channels. The amp is connected to the speaker via a series 10 Ohm resistor. The right channel is connected to the amp output directly (source voltage). The left channel is connected to the speaker terminals after the calibrated 10 Ohm resistor (impedance voltage). You then set the loaded output to between 1v and 2.8v at 400hz. The program then measures the difference between the two channels while driving it with Periodic noise or a heterodyne stepped sine sweep to get a constant voltage impedance measurement. I'd rather use a constant current method but I haven't figured out how to hook it up and calibrate yet. Since these cards usually clip at around 500mv the output from the amp has to be carefully set so as not to drive the input too hard and mess up the measurement, which is where having a concomitant oscilloscope running on the same input really helps.

I purchased an external USB card as a replacement and it measures cleaner than the original realtek. I still need to run Rightmark on it but it will work fine. I just need to figure out why the amp output has so much voltage in relation to the laptop input. It acts like the two devices are ground reversed which could expose the sound card input to wall socket voltage. I'll set it up again and measure AC and DC voltages between the inputs and amp outputs to see what I get then report back. It can't be that big a difference or I'd have stopped my heart by now. I just wondered if anybody else has run into this situation, thanks.