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Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
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Old 11th December 2017, 12:00 PM   #41
suzyj is offline suzyj  Australia
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Quote:
Originally Posted by udok View Post
Of course with the removed silly 22nF input capacitor as you need
a fast risetime in Spice to excitate oscillation!

Don't forgett that in a real cirucit the thermal noise will excitate
your circuit from DC up to infrared and will trigger any instabilities...
And how exactly will thermal noise generate HF at the millivolt level that you're putting into the front end? input referred noise for this is likely to be better than 10nV/sqrtHz. There's just no way that is going to provide enough excitation.

I have no idea why I'm even humouring you, given you're being incredibly rude and aggressive, but here's a version of the compensation (exactly the same amplifier) that'll put up with your square waves with no input HF filter, not to mention being reasonably stable down to around 6dB gain. There's a penalty for that though - 10KHz THD has blown out past my 1ppm design goal.
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Old 11th December 2017, 01:25 PM   #42
TNT is offline TNT  Sweden
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Originally Posted by AndrewT View Post
It depends on what distortions are being generated.
An amp that has zero crossover distortion and ~0.1% of 2nd and 3rd THD will probably pass your test.

An amplifier that has crossover distortion and a very low proportion of 2nd & 3rd can sound terrible at the 0.1% level.

HF components of distortion from 5th to 25th that are very short term but high in amplitude will read low on a conventional THD meter. But it's the high amplitude (low duty cycle) that we can hear and dislike.
How does x-over dist differ from 2nd and 3rd order?

/
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Old 11th December 2017, 01:42 PM   #43
AndrewT is offline AndrewT  Scotland
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Cross over distortion (ClassAB) is physically very different from low order distortion (ClassA) and it sounds very different.
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Old 11th December 2017, 02:09 PM   #44
udok is online now udok  Austria
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Great, you have finally added reasonable compensation caps

Your amp is now stable with about 20 degree of phase margin, though
on the border.
Try to add 10 nF to the output - it will again burst into oscillation.

No wonder, this beast has 107 dB open loop gain @ 10kHz!
And the open loop response does not look as if someone has
spend time on it.

Thermal noise is amplified as any signal,
and the output noise couple back to the input through the feedback resistors.
If the amp is unstable it will add in a positive way,
and in a few mS later you have an oscillator.

Besides the stability problem, the amp has other serious shortcomings:

It wastes 1/3 of the supply power with 4 Ohm load as the
diamond-buffers have serious shortcommings in power amps:
They limit the base current of the output transistors.
=> Bad peak power capability => forget them in this config.

The DC level of the first cascode stage is still rather random. It needs
a common mode stability circuit.
A current mirror would be a very good idea, but this does not work
in your amp.

The nice idea is that you have put 3 amplifier stages in one amp.
The bad news is that you have not though about it.

Look up some text about opamp design. The better ones have
three stages and some clever compensation schemes.

And forget your 1ppm goal. At this low levels simulation will tell
you exactly nothing as layout and transistor effects not modelled in
LTSpice are more important.

Quote:
Originally Posted by suzyj View Post
And how exactly will thermal noise generate HF at the millivolt level that you're putting into the front end? input referred noise for this is likely to be better than 10nV/sqrtHz. There's just no way that is going to provide enough excitation.

If i seem to rude, i apologize. At least i have spend my time to try to understand your beast.

I have no idea why I'm even humouring you, given you're being incredibly rude and aggressive, but here's a version of the compensation (exactly the same amplifier) that'll put up with your square waves with no input HF filter, not to mention being reasonably stable down to around 6dB gain. There's a penalty for that though - 10KHz THD has blown out past my 1ppm design goal.

Last edited by udok; 11th December 2017 at 02:17 PM.
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Old 11th December 2017, 02:15 PM   #45
PB2 is offline PB2  United States
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Quote:
Originally Posted by suzyj View Post
I'm also playing with 3 sets of BD179/180 in lieu of the TIP41/42, simply because it allows me to spread the current source load across 3 transistors, and I can then use 2N3904/3906 rather than the BD139 there (higher gain), which in turn reduces current requirement for my reference.
Have you considered the BD139/40 -16 which are higher gain?

I consider part of the challenge being to keep it simple(r) simply because
increased complexity adds to build time and chances for error - in this case.

Also, I would not use off brand dirty cheap semis for fear of fakes, not sure
what you had in mind.
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Old 11th December 2017, 02:15 PM   #46
TNT is offline TNT  Sweden
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Originally Posted by xrk971 View Post
....d work just fine and you won’t hear the difference in the 1ppm THD vs 0.01%THD. These sound great (super bass punch and nice mods and highs). Also, they are single rail and can do up to 25w easy. They are the size of a large postage stamp and you can run them off cheap 19v smps.

35W TDA8932 Digital Amplifier Board Module Mono Low Power Stereo Amplifier | eBay
And the drop after 8khz - is that the measuring filter that is allowed for class D-amps i.e. in reality, the distorsion keeps increasing with freq?

//
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Last edited by TNT; 11th December 2017 at 02:17 PM.
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Old 11th December 2017, 09:03 PM   #47
anatech is online now anatech  Canada
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Cascading diamond buffers - a cheap low THD 10W amp with TIP41C

udok,
Tone it down. I have to agree with suzyj about your posting style. You aren't adding anything of value if you continue to post this way.

If you can't post anything of value, consider spending time in another thread - Nicely!

-Chris
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Old 11th December 2017, 10:22 PM   #48
Bigun is offline Bigun  Canada
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Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
Quote:
Originally Posted by udok View Post
Great, you have finally added reasonable compensation caps

Your amp is now stable with about 20 degree of phase margin, though
on the border.
Try to add 10 nF to the output - it will again burst into oscillation.

No wonder, this beast has 107 dB open loop gain @ 10kHz!
And the open loop response does not look as if someone has
spend time on it.

Thermal noise is amplified as any signal,
and the output noise couple back to the input through the feedback resistors.
If the amp is unstable it will add in a positive way,
and .....
There’s good advice here, high feedback amps work great in Spice but need lotsa care in real builds. The promise of low distortion is hard to achieve but there are some good examples on the forum. Parasitic are critical and not included in basic Spice simulation. I’ve respun pcb’s before because of this.

I also agree with the futility of believing extreme low distortion from Spice. The device models are not that accurate, passive portrayal often idealized. Spice is helpful to explore changes, tolerances, doing some sensitivity analysis on key parts parameters.

My best amp simulates with low distortion, I aim for <.005% but I have also enjoyed amps with higher distortion and I will continue to build different types.
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Last edited by Bigun; 11th December 2017 at 10:28 PM.
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Old 11th December 2017, 10:42 PM   #49
PB2 is offline PB2  United States
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Quote:
Originally Posted by Oldvinylplayer View Post
Oh dear. Now I just have to get a move on with the line arrays.

Given that this is a ground up design, is there any advantage in making a higher voltage amplifier to drive more than one TC9 in series?

Graham.
It is better to drive them in parallel since lower load impedance requires lower
rail voltages keeping the outputs out of secondary breakdown.
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Old 12th December 2017, 12:02 AM   #50
xrk971 is online now xrk971  United States
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Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
Quote:
Originally Posted by TNT View Post
And the drop after 8khz - is that the measuring filter that is allowed for class D-amps i.e. in reality, the distorsion keeps increasing with freq?

//
They probably only show data out to 8kHz because distortion is predominantly 2nd and 3rd harmonic, so 8kHz x 3 is 24khz and data acquisition may only be 48khz.

The amp I linked is a mono PBTL setup where both channels drive one load. The current would be half, so I think distortion is less than plot which is for BTL stereo load.

I should measure it driving 8ohm load to 10w for grins.
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