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Old 7th November 2011, 05:25 AM   #1331
Jay is offline Jay  Indonesia
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Nice work Ruwe! I like it!
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Old 7th November 2011, 06:44 AM   #1332
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Quote:
Originally Posted by Ruwe View Post
That's all so far. The amp sound is very good, but I'll give it more listening once I build decent boards and put it in an enclosure. The original version sounds kind of too warm and one-note in the bass to my ears. My definition for good sound is detail and definition, but not on the "clinical" side. The type of 300B single-ended comes to mind. I think this circuit has the potential to be close.
If you are interested, Nelson Passes' F5 article (manual) and his Super Symmetry article are very interesting reading.

Regards
Ruwe great job on a SSA Basic version you did, congratulations. My basic version differs in a way that I always used BIGBT's as outputs, so having lower output impedance and probably less issues than in your's really basic version. It can't be even simpler than this as Elvee suggested to omit cascodes without significant change, than you've had only four transistors in your case. Well I don't know if that would work with the same bandwidth and same stability.

But what Elvee found out from his simulations is lowering the overal distortions from 0,3% to 0,07% if there are no 10 ohm emitter resistors present in the input stage. So my suggestion would be to check the optimum values of these two resistors to find the best correlation between harmonic distortion profile and best sound of this basic version. If you like you can even check what happen if you omit casodes. Of course you can leave it like it is now, it is good sounding simple amplifier anyway.

Thanks Ruwe for your contribution to this thread and DIY audio community, your info is of a great value to all of us, regards Andrej

Last edited by Lazy Cat; 7th November 2011 at 06:48 AM.
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Old 7th November 2011, 06:59 AM   #1333
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Quote:
Originally Posted by Ruwe View Post
I had to discard the use of vertical MOSFETS and BJTs, because of their high temperature drift.
They need thermal compensation, for sure.
Quote:
Originally Posted by Ruwe View Post
1. Circuit gain was not constant with the original values in the feedback. It was dependent on the output load and the output amplitude. ...
4. In the original SSA, with one channel loaded at half power, I noticed that the other channel changed its otput DC component from 10mV to, say, 200mV. ...
Let me think you have a poor PSU ?
btw: feedback resistances needs to be at least 3W.
Quote:
Originally Posted by Ruwe View Post
My SSA version has 4MHz oscillation that tends to show itself with some speakers and cables. ... I added capacitors for stability.
I believe the issue can be better addressed those ways:
- Accurate shield of Amp & PSU.
- Is your R5/R6 soldered right at the gates ?
- Did-you use some Selfic resistances somewhere ?
- Avoid Parasitic caps in the Feedback ends (Did your response curve show a peak between 4 & 10 Mhz ?)
- Playing with some Boucherot cell (Snubber) ?
- Following your needs, and if you drive very capacitive loads, i would consider a self+R in // in serial of the output (Tank ? = circuit 'bouchon' in French).

It is a pity to reduce too much the bandwidth. I would try to keep-it flat up to 2Mhz. Here too the supply impedance at high frequency and correct decoupling with film caps on the PCB can have a huge influence.
The first thing would be to try to get your amp clean without C in CR. Then, tune one //C in the feedback loop just enough to get a flat response curve at HF if needed. Then a low pass filter in the input to kill any overshoot on square waves. Just at the limit for each step.
If you understand French (or can use some Gougeule translation), that is the ways i had addressed those issues on my SSA amp:
www.esperado.fr - Le crescendo revisité
and the power supply:
www.esperado.fr - Alimentation Crescendo
My two cents.
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Last edited by Esperado; 7th November 2011 at 07:18 AM.
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Old 7th November 2011, 07:10 AM   #1334
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... and Ruwe one more thing regarding stability issues and consequently the amp's bandwidth (omiting 220pF caps), bases of cascode transistors needs 1uF to GND for HF stability, ELCO itself doesn't provide low Z in MHz region. Using 1uF decoupling in all DC points helps stability in this amp more than you can think of. Also use 1000uF instead 220uF in parallel with zeners.

Hi Christophe, thanks for your contribution too.
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Old 7th November 2011, 10:04 AM   #1335
Salas is offline Salas  Greece
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Ruwe had very strong DC problems initially and he worked a lot to make it viable. Nice. To have less THD it would take to add drivers of some kind is one good plan I think. Omitting 10 Ohm will help THD but let Iq wander more with temp? Andrej, until getting some LM334, what do you think about JFETs as current feeds? I got some that take some time to reach top IDSS with temperature, if I blow over them they lose some. Benefit is they are extremely quiet, when LM334 datasheet says it will add 12dB of noise. But can such JFETs help our input transistors Tc thing?
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Old 7th November 2011, 11:28 AM   #1336
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Hi Salas

Drivers and source follower output mos-fets would certainly lower the THD, but in a simplest version like this is, optimization (not complete absence) of the Re's values can bring some improvement - balance between THD & sound quality. Definitely worth to try in this phase of the construction Ruwe is now.

Current feeds to the bridge with j-fets is a good idea because of their negative tempco, similar temperature dependency like input BJT's, only reverse. It would be a simple one j-fet solution per 15 V rail, of course adjustable to optimum current value with gate-source resistors. BF245B seems to be appropriate type, 6-15 mA drain current, HF, low noise, ... If this solution would prove to be adequate, than you can simply omit j-fet from your VAS stage.
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Old 7th November 2011, 11:46 AM   #1337
Salas is offline Salas  Greece
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I have those BF245B. Will try along this test schematic lines.
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Old 7th November 2011, 12:02 PM   #1338
Salas is offline Salas  Greece
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P.S. To explain a bit, this one runs half current on input transistors than before to keep them cooler and hopefully tamer, hence the 1K cascode loads that were 470R before. The drivers are BD140/139 still, just the spice models are from NXP same die surface mounts. Output bias target is still around 175mA, that spice predicted very well before I must say. With the current feeds there is 5dB more OLG predicted also.
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Old 7th November 2011, 08:14 PM   #1339
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Exclamation SSA Basic MOS-FET

SSA Basic MOS-FET version with J-FET current sources to provide supply current into SSA feedback bridge, serves to compensate collector current thermal drift of the input pair and consequently the bias of the whole amplifier.

This version was inspired by Salas, based on Nico's mos-fet SSA amplifier from early of this thread.
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Old 7th November 2011, 08:56 PM   #1340
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Exclamation SSA Basic Easy

Quote:
Originally Posted by Ruwe View Post
I know most of you are trying more complex schematics derived from the simple one, but I am too stubborn or just silly, and wanted to make the original simple one to pass my usual amplifier tests.
SSA Basic Easy simplest of them all, only four BJT's, inspired by Ruwe, based on Elvee's suggestion.
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