valve frequency responce

[steve s]

Do you sell the transformers ? UK made high quality transformers are in short supply. Sowters are good, but thicken the bass up a fraction in my limited experence I would be very happy to try a pair of yours. Could compare them to the tributes I use.

No sorry we dont, our transformers are made strictly for our amps and designed for each unit, thats one of the main reason the amps sound like they do.
We also do a valve preamp which does dc to 14mhz we couldnt test further as the signal generator starting rolling off.

That's a shame, my experience is that transformers are made to match the impedance's
it's a shame your not putting your expertise into some thing many of in this forum would want.. Good quality output transformers... most of us are like yourself, we make our own amps.
I'm aware they are challenging to make, most colour and are usually not as transparent as one would hope, a UK high quality supplier would be an asset to me.

[BobWeaver]

Recent project, low power (700mW) amplifier (single 6M11 3 section valve, pentode preamp and dual triode P-P output), I was amazed to get 30Hz to 80kHz (and still climbing). I was just twisting the frequency knob on the signal generator, and was about to switch to the next higher range when I remembered that the speaker was connected, and that it might not be enjoying these kinds of frequencies. So I stopped at that point.

Most surprising is that the output transformer was just a cheap 70 volt line matching transformer. However, I was using a lot of negative feedback. So that likely helped.

[Paul Barker]

Sweet little project.

[Paul Barker]

Paul is starting to look right.

I may have sparked a self fulifilling prophecy. OP may have originally not intended to use the thread as a promotion platform. We will never now know. If I had not made my prediction and he had still promoted his products I could then have been a prophet. But is a prophet who keeps quiet still a prophet?

Next time I'll write it in an envelope photograph it with the daily newspaper and we can open it at an eggfest

[Mike H]

Most surprising is that the output transformer was just a cheap 70 volt line matching transformer. However, I was using a lot of negative feedback. So that likely helped.

Yes it should, but also the transformer likely has some HF resonance which would help explain it as well, to find out what that is try it without the negative feedback.

[BobWeaver]

I did the original design and set up without feedback, and it was reasonable frequency response, if I remember correctly (but I often don't).

I'll pull the NFB and have another look. More later...

[Mike H]

You should use a dummy load resistor not a speaker for a freq sweep test, as a speaker is a dynamic load and you can't see what's really happening. The appearance of "good" up to 80 kHz is most likely due to voice coil reactance being tens of Ohms at that point. In fact most speakers cease to be 8 Ohms by the time you get into just the mid-range area.

With a purely resistive dummy load, vagaries of transformer behaviour really show up.

[BobWeaver]

I dug out an 8 ohm power resistor, and gave the amplifier a frequency sweep. All numbers are relative to 0 db at 1kHz.

Open loop, it was fairly flat to 10 kHz and then dropped off steadily until it was down 3db at 15.9 kHz. I continued to sweep up to 100 kHz, but there were no resonances. The output just dropped steadily. Distortion in the output sinewave was only visible by superimposing the input and output scope traces. So, even without NFB, distortion was quite low. Just a slight flattening of the top of the sinewave.

With negative feedback connected, output was flat to about 12 kHz, then started to rise very gradually and steadily until it was up about +0.8 db at 30 kHz. Above 30 kHz, the amplitude started to decrease and the waveform started to become slightly distorted (2nd harmonic), but straightened out into a sinewave again by about 50 kHz. Output was finally down 3 db at around 70 kHz. In the audio range (<20kHz) distortion in the output sinewave was not visible even when the input and output traces were superimposed on the scope. Phase shift between input and output was barely noticeable at frequencies below 20 kHz, but naturally increased at higher frequencies (and below 100 Hz).

So, my earlier estimate of 80 khz was overstated. It appears that the dial on my signal generator is not as accurate as I thought. This time, I used a frequency counter when I did the tests. Also, the previous numbers were no doubt influenced by having a speaker connected rather than a resistive load.

The slight distortion between 30 and 50 kHz is interesting. I'll have to look into it a bit further, but it's of no real concern, being at that frequency. (I don't buy into the argument for frequency response above the range of human hearing.)

[Mike H]

No I never trust dials as being very accurate, unless the equipment cost lots and lots of money in which case they should be reasonably accurate

In the latter case the 'better' freq. response is entirely due to the NFB, as it allows the amplifer to work harder than otherwise (gain increases) to keep it up at the top end. However this means much more distortion at the top end also.

The peak is due to phase shift introducing positive feedback, most probably, or could be the OPT's natural resonance. Fortunately the early roll-off of that particular transformer prevents the amp going into parasitic oscillation gymnastics.

All in all the early roll-off without NFB and the phase shift indicates relatively poor coupling between the windings of that particular transformer.

A second logical step might be a square wave test to see what the overshoot and ringing is like. This should indicate the natural resonant frequency of the transformer (the ringing freq.).

[BobWeaver]

I wasn't really expecting much from a $12 transformer. I guess my main point was that I was surprised to find it to be as good as it is.

[robert]

Interesting topic, I would like to know what audible effect would ringing
have from a audio transformer, IE would you hear it as distortion or some other form of noise.

Robert

[Nick]

Interesting topic, I would like to know what audible effect would ringing
have from a audio transformer, IE would you hear it as distortion or some other form of noise.

Robert

In theory as long as the resonant frequency was out of the audio band, then none worth worrying about.

I would only worry about it if the amp used feedback, as it could combine with phase shift in the feedback circuit to cause HF instability.

Other than that, trying to damp it can end up doing more audible damage that it fixes, it may look nicer on a scope, but sound worst.

[Mike H]

[BobWeaver]

1kHz Square wave response:
In each photo, upper trace is input from signal generator. Lower trace is output across 8 ohm resistive load.

Either with or without NFB, there's no sign of any ringing. Again, response seems pretty good to me, considering the cheap transformer. But of course, we're talking quite low power, so that makes things easier. I've reduced the amount of NFB from what is shown in the previously posted schematic, because I've added a tone control, and needed to increase gain to compensate for tone control insertion loss. NFB resistor was increased from 680 ohms to 3.9k. Tone control is disabled for these tests.

[Mike H]

Very interesting, totally unexpected. Have you still got the pot and 680pF across the input?