Mod your Beresford 7520

[leo]

Right, with the original op-amp fitted, I get almost the same shape waveform as with the 4920 but with some extra distortion at full volume, so this is the worst so far. Interestingly I get an almost perfectly smooth waveform from the fixed output, so It seems to be the variable output that is distorting the signal.

I'll do more testing tomorrow night. Let me know if you think of other things for me to try. Oh and I'll try and take some photos of the scope display to make it easier to see what I'm going on about.

If the fixed output is ok but the variable output is clipping at any setting on the pot then the actual op-amp gain may need reducing, should just need a few resistors changing

[leo]

Leo 6v P to P is just a smidge over 2v rms The same as 99% of CD players and DAC's

Brain fart

[ReachtheSky]

Thanks Krisbee.
That's odd, I was expecting it to be higher than that on the preamp output as I can't take my volume level much above 1/4 without it being too loud and my power amps are supposed to make maximum power from a 2.3V input. If I go above half volume with no music playing I get odd buzzing sounds from the speakers as though I'm over-powering the power amps input or something. I'll have to investigate further.

The preamp output from the 7520 at 1.8V is a little less than most CD/DVD players output at about 2.0V.

I’m using the 7520 preamp output into mono block power amps powering B&W801 monitors and I can confirm that it works beautifully. The background is particularly quiet, almost silent at max volume. I suspect you may have a problem elsewhere or a faulty component somewhere?

[HighFidelityGuy]

Hmm, this is very odd then.

My monoblocks make full power at 2.23V and have an input impedance of 50K Ohms.
I don't know whether that means they are a good or bad match for the 7520's variable output. Does anyone know?

[leo]

It should match ok, apart from the crackling noises and measured clipping on the variable output is everything else ok?

[HighFidelityGuy]

I've not had any similar problems with anything else. If I run the power amps off my AV amp I get plenty of range on the volume control (on the AV amp), no crackle, no pinging noises from the DAC.

I only get issues if I run the power amps directly off the DAC. Also, like I mentioned earlier, the fixed line output gives a clean signal, the variable output is distorted but that's only noticeable on the scope, I can't hear any distortion. It's as if there is something wrong with the volume pot or related circuitry between the op-amp and the variable output.

Any ideas?

[leo]

As I don't have a 7520 I can't be sure how its run and can only give an idea to try and help.

From what your saying I get the impression the gain of the variable output section is too high and maybe this was highered during production to make it better suited to a wider range of headphones including high impedance ? if the headphone amp part also serves as a line stage then maybe its just too high for some amps input?

If I had this problem I'd lower the gain setting by adjusting the resistors around the op-amp in the variable output section, this would probably eliminate the clipping your getting on the scope and would definitely give a wider range on the vol pot if that makes sense?

[Dougr33]

Stan should give Leo a 7520 so he can be even more helpful!

[Dougr33]

Help! Is anyone having this issue? I took out my 7562s and put in the 4032s (new Tirna version). Variable out to my amp sounds fine. But the headphone amp sounds noisy/crappy/distorted thru the first 1/6 rotation (lower volume). I have the 50ohm Sennheiser 595s.

The HPA sounds fine if I... 1) remove the variable out 4032 and leave empty, 2) replace the variable out 4032 w/ the 4562, or3) use a 4562 back on the HPA with a 4032 on the variable out. Both 4032s sound the same (i.e. either works fine alone in the HPA, but as soon as two of them are onboard, the HPA sounds like crap).

Anyone else??? This is a bummer.

[leo]

I wonder how many people are running it using the same op-amps and not realizing theres a problem if their not using headphones