DIY Transistor Amp Using NVA Boards

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Nick
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#301 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

On thing I had to do with the amps I made was add a HF detection leg to the speaker protection. Mainly to stop me from burning out the resistor in the zobel. When running for testing with the low pass on the input removed the amp is happy to run at 100W at 200kHz (or more), of course at that point the 10R resistor dies :-)
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#302 Re: DIY Transistor Amp Using NVA Boards

Post by JamesD »

Quick aside... RE's compound pair is the Sziklai or complementary pair that I like a lot but it is prone to burst oscillation in its simple form as it has multiple internal feedback paths with lots and lots of gain and very little damping on its hf behaviour - I have found it can be stabilised by a small resistor between first and second transistor with the normal hf limiting cap between collector and base of the second transistor - it didn't work for me when the cap was on the first transistor but then I wasn't using GFB and this cap would be more effective in that case. LTSpice models the behaviour and stability or otherwise quite accurately if you extend the bandwidth sufficiently and include the relevant parasitic reactances...including the pcb and psu effects as gain can be positive into the 100s of MHzs so model to 1GHz (and as a further aside model down to 0.001Hz with an extended simulation period of, say, 20-30 seconds as Mike recently mentioned too).
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#303 Re: DIY Transistor Amp Using NVA Boards

Post by Daniel Quinn »

But I’ve never had an empirical problem , Elliott’s comments are thus for me purely theoretical .

And I’ve purchased about 20 different boards and used the same power supply to run them all .
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#304 Re: DIY Transistor Amp Using NVA Boards

Post by Ant »

Those boards may have had some sort of zobel or similar network on them anyway
We know richards dont but the others are unknown unless you traced them or had a circuit diagram to look at and recognise a zobel or lack therof
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#305 Re: DIY Transistor Amp Using NVA Boards

Post by Daniel Quinn »

Some may , some definitely didn’t .
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#306 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

A few observations:
There is a definite improvement in sound quality, with the combination of Zobel across the output and series connected, parallel resistor/inductor.
The amp has taken on the silky, liquid sophistication of a big, high end 1970s Japanese amplifier. The similarity in sound to one of those old behemoths is quite uncanny and personally I love it. But why has this happened?

My opinion (that’s all it is) is that without the compensation networks, the output stage could have been oscillating, ringing or both, under drive conditions but stable with no signal. Elliot does in fact mention this possibility at the end the extract I quoted

The choice of inductor size is not difficult - for an 8 Ohm load it will be typically a maximum of 20µH, any larger than this will cause unacceptable attenuation of high frequencies. A 6µH inductor as shown in Figure 10 will introduce a low frequency loss (assuming 0.03 Ohm resistance) of 0.03dB and will be about 0.2dB down at 20kHz. These losses are insignificant, and will not be audible. In contrast, ringing (or in extreme cases, oscillation) of the output devices will be audible (even at very low levels) as increased distortion, and in extreme cases may destroy the transistors

OTOH it could have been RF frequencies from switching power supplies, resonating with the tuned circuit formed by the speaker leads and internal stray capacitances within the amp. As below:

It is a simple fact of life that an emitter follower (whether Darlington or compound) is perfectly happy to become an oscillator - generally at very high frequencies. This is especially true when the output lead looks like a tuned circuit. A length of speaker cable, while quite innocuous at audio frequencies, is a transmission line at some frequency determined by its length, conductor diameter and conductor spacing.
All power amplifiers (well, nearly all) use emitter follower type output stages, and when a speaker lead and speaker (or even a non-inductive dummy load) are connected, oscillation often results.


Increasing the feedback levels by 6dB a while ago seemed to improve things but adding both the networks has lifted another layer of hash to give the open, neutral, sophisticated sound I’m hearing now. Our Ant came round yesterday and commented on how great the Mission speakers were sounding with the amp. Within the limits of the close-to-the-wall speaker placement, there is better lateral placement of individual instruments in the stereo image. The front to back depth is much better and the overall presentation is refined and lifelike.

At idle, ear to speaker, there is absolute silence, whereas before adding the networks there was a slight hiss mixed with a continuous background ‘zzizzzz’ going on, which could have been the RF pickup mentioned earlier.

I might have had output stage ringing and RF pickup via the speaker leads and the Zobel cured one and the inductor/resistor cured the other. Without testing I don’t know and it’s a bit warm for that sort of thing at the moment.
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#307 Re: DIY Transistor Amp Using NVA Boards

Post by Mike H »

I remember a trick from somewhere, a 10 Ohms resistor with some enamelled copper wire wound around it.
 
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#308 Re: DIY Transistor Amp Using NVA Boards

Post by jack »

Mike H wrote: Tue Jul 19, 2022 12:41 pm I remember a trick from somewhere, a 10 Ohms resistor with some enamelled copper wire wound around it.
Used to be pretty common.
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#309 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Still is.
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#310 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Mike H wrote: Tue Jul 19, 2022 12:41 pm I remember a trick from somewhere, a 10 Ohms resistor with some enamelled copper wire wound around it.
Yep, that’s exactly what I did.
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#311 Re: DIY Transistor Amp Using NVA Boards

Post by Mike H »

There you go then! Well I can tell you nothing can I? :lol:
 
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#312 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

You’ve forgotten more about solid-state than I know Mike. :lol:
I’ve only just started with the knowledge.
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#313 Re: DIY Transistor Amp Using NVA Boards

Post by Mike H »

:lol:
 
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#314 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

NVA based SS amp is now a permanent fixture:
2891FE90-764B-4323-88E4-480681E83264.jpeg
2891FE90-764B-4323-88E4-480681E83264.jpeg (124.22 KiB) Viewed 3740 times
With 85dB efficient speakers, a solid-state amp of decent power is the obvious choice.
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#315 Re: DIY Transistor Amp Using NVA Boards

Post by karatestu »

Cressy Snr wrote: Sun Nov 06, 2022 10:08 pm NVA based SS amp is now a permanent fixture:
2891FE90-764B-4323-88E4-480681E83264.jpeg
With 85dB efficient speakers, a solid-state amp of decent power is the obvious choice.
Looks easy on the eye Steve. I expect it's kind to the ears too. :D
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