Mod your Beresford 7520

[Krisbee]

I asked the same question of Mr.B himself. The answer is: "the TC-7520 can output up to 1.8V on the line and the preamp output".

[HighFidelityGuy]

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.

[leo]

Afraid I can't help much with a lot of questions for the dac

Did you get this noise with the original op-amps installed?

[HighFidelityGuy]

No, I didn't have my power amps when I had the original op-amps installed so this is the first time I've used the variable outputs.

I did some testing when I got home tonight and it seems that I got it a bit wrong, it's not so much a buzzing, it's more of a crackling. Nothing too serious.
I also did some testing with an oscilloscope and got some interesting results.

Using a 50Hz test tone (all I had handy) I'm getting 200mV peak to peak from the variable output when the volume is at approx 1/4. The signal also looks a bit messy.
If I crank the volume up to full, the output goes up to 6V peak to peak. The fixed output is also 6V. At this level the signal looks super clean.

So that would explain why it's really loud for me above 1/4 as my amps are 550W, so even at 200mV they will be kicking out a fair amount of power.

I'll try and put the original op-amp back it to see if this gives the same results.

[leo]

6v! Crikey thats some output

For general testing I use 1k at 0db http://binkster.net/extras.shtml .
For 50hz the pot may be having some effect on what your seeing on the scope

[leo]

BTW, can you get get a reading on your scope of this crackling noise?

[leo]

Another thing, I'd try the original op-amps and then see if you get the crackling

[HighFidelityGuy]

Ok, I've made a 0dB 1KHz test tone and done some more testing. I get slightly over 6V peak to peak with this on the 4032 op-amp and slightly over 5.5V with the 49720 installed.
I still get lots of fuzz on the waveform at low volumes but with 1KHz I also get some distortion to the peaks as well. This didn't happen on the 50Hz tone. The peaks are getting squared off slightly on the left side. This is on the 4032. The 49720 is even worse. The peaks look almost like a saw-tooth wave. This can't be good.

I'll try the original op-amps next.

Edit: I forgot to mention, I've not managed to get the crackling during testing. This only seems to happen through the amps, so I think it's because they're getting too much voltage. Also, I've noticed another odd problem: While playing the test tones, I sometimes heard a quiet ping come from DAC it's self. When this happend the wave form on the scope glitched like the DAC output was being cut off for a split second. This also can't be good.

Edit 2: For clarification; the squaring of the top of the waveforms didn't just happen at full volume, it was at any volume. So it's not the output clipping I don't think.

[HighFidelityGuy]

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.

[NRG]

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