6ck4 pp amp

[EC8010]

OK, I've downloaded, installed it, and had a quick look at it. It seems to recognise and be happy with my M-Audio 24/192 card, so I'll have a little play with it tomorrow.

 

[SY]

It works quite well if you've got two channels with roughly unity gain. Here's a spectrum of the Heretical passing a 1V sine wave.

 

[SY]

And here's the IM with 10 and 11 kHz. Because of unavoidable test modalities (fancy way of saying that there's too many wires and they're all too short), the DUT was situated between two active power strips and sitting on another active line cord. Thus the lousy 60 Hz result...

 

[Sheldon]

What I'm getting at is that you shouldn't be seeing that cluster of sidebands spaced 120Hz(?) away from the 1kHz fundamental and its harmonics. In my experience, that indicates a poor power supply, and often that the amplifier is going into Class B. That's why I wondered about what power level you had.

But you've lost me by saying that the output isn't a steady tone. Am I misinterpreting your images? I had assumed that your THD spectrum was the result of applying a steady 1kHz tone and then doing an FFT on the output of the amplifier?

On second thought, I've gone back and redone my measurements. I discovered that I had wired my RCA wrong, which caused some grounding "issues". I redid the measurements with an 8ohm resistor. The THD and IMD came out much lower. The fr was better too. I checked the output during the test and it's around 0.5volts for the steady tone portions - can't tell for the others.

Also, I noted that I got somewhat better results when using the battery for the laptop, instead of the wallwart.

Frequency response: White line, amp Green line, card

 

[Sheldon]

noise

noise

 

[Sheldon]

THD

THD

 

[Sheldon]

IMD

IMD

 

[Sheldon]

What I'm getting at is that you shouldn't be seeing that cluster of sidebands spaced 120Hz(?) away from the 1kHz fundamental and its harmonics. In my experience, that indicates a poor power supply, and often that the amplifier is going into Class B. That's why I wondered about what power level you had.

But you've lost me by saying that the output isn't a steady tone. Am I misinterpreting your images? I had assumed that your THD spectrum was the result of applying a steady 1kHz tone and then doing an FFT on the output of the amplifier?

I've gone back and done a little more testing, and understand the system better. I assume that the THD test is a steady tone. I was confused by the fact that RMAA does a series of tests if you set it up that way - a different tone for each test. Anyway, yes those sidebands are still there and are at 60hz intervals (1060, 1120, etc.,). The higher peaks are the 120hz at -63db for 120 and -80 for 240, declining quickly after that.

Interestingly, I used the test to check various tubes. Sy is right about thee 12BZ7's. Of 5 that I have, three give signicantly more high order harmonics than 12AX7's. Two of the 5 were pretty good, but still not as good as 12AX7's or 5751's. The lower harmonics were pretty similar. Actually, the best overall seemed to be 12AT7's, even though they are well out of their usual operating range. The 5751's had very slightly better distortion but with a little more high frequency roll-off. Another interesting note: I tested two different AT7's, some NOS Telefunken's I bought a couple of years ago (they are ridiculous in price now) and some select Golden Dragon 's that have been running in my main amps for a couple of years now. Virtually identical, but one of the Tele's was a little noisier and the Tele sections were not as well matched.

AT7 results (1.5v at 8 ohms), -.5db at 20hz, -.5db at 20khz

2nd, -55db
3rd, -68
4th, -107
5th, -91
6th, -110
7th, -107
very little higher

5751 (same conditions), -.5db at 20hz, -.9db at 20khz

2nd-58
3rd -63
4th -98
5th ---
6th -109
very little higher

Sheldon

edit: Oh, EC8010, if you're still following this, any suggestions on improving the PS, short of a complete redesign. Better caps, etc.? It's obviously very basic. Not much room in there.

 

[EC8010]

I went back to the first post. You gave a schematic showing that you simply have a rectifier connected to a reservoir capacitor feeding the output stage. I would suggest that you interpose a section of LC smoothing between that and the output stage. Off the top of my head, I'd suggest a 5H choke and a 100uF capacitor. Even better, model the thing in PSUD2 and find out optimum values - you might find that it helps to reduce the value of your reservoir capacitor. (That's why I'm suggesting that you use 100uF in your LC section - you can use your existing capacitor.) Bolt the 5H choke on the outside of the chassis...

 

[Sheldon]

Thanks,

Seems reasonable. I'll order a choke next time I get parts and give it a shot. Maybe I can even stuff it in there somwhere.

Sheldon

 

[Sheldon]

Turns out I have a 1.5h, 200ma choke that I had ordered some time ago by mistake. Put that in before the 100u cap with no other changes. Voltage dropped from 355 to about 348. Considerable improvement in sideband noise. This is about 1.6V into 8 ohms, at least that's what the DMM reads during the highest test signals.

Sheldon

 

[Sheldon]

before choke

 

[EC8010]

That looks a lot better. Add a reservoir (50uF?) before the choke and see the hum really drop. Does it sound better?

 

[Sheldon]

That looks a lot better. Add a reservoir (50uF?) before the choke and see the hum really drop. Does it sound better?

I tried that. A couple volts higher, but no significant change in the spectrum. In the existing supply there is a 47uF choke, but it has 150ohms in series to ground for the CCS supply. Adding an extra 47 doesn't seem to make much difference as noted.

Sound better? I think it sounds a little clearer overall. But I haven't listened much and I don't trust my memory for a good A/B. Overall, it is a nice sounding little amp as long as it's not driven too hard. I do like that it measures better. I'm in the middle camp. I don't think zero distortion in the output necessarily makes for better sound. I can believe that a little low order harmonic distortion might be dandy, but I doubt PS distortion generally does any good for the sound.

Sheldon

edit: BTW, I'm having fun taking these measurements with RMAA. It's nice that I can plug in various tubes and see different noise and distortion patterns. Makes it easy to select tubes for that specific circuit, and the best position for a PP pair. I can even watch the changes as I fiddle with bias or balance.