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Alpha Nirvana 39w 8ohm Class A Amp
Alpha Nirvana 39w 8ohm Class A Amp
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Old Yesterday, 03:24 PM   #171
jacques antoine is offline jacques antoine  France
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Alpha Nirvana 39w 8ohm Class A Amp
Quote:
Originally Posted by xrk971 View Post
This board is going to be a joy to build. Very compact and easy parts placement. Not that many parts either.
Hi X,

I noticed several contact points for SMD components on the main and mini boards.
While I understand the pleasure to make the circuit as small as possible I guess full TH is no longer an option for the final PCBs ?
Cheers,

Jacques
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Old Yesterday, 04:07 PM   #172
Vunce is online now Vunce  United States
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Quote:
Originally Posted by xrk971 View Post

Itís amazing how compact it is. Under 4in per edge.
They almost resemble the look/shape of a Ďchipampí type of amplifier board.
I like how the helper pcbís turned out
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Old Yesterday, 04:22 PM   #173
xrk971 is offline xrk971  United States
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Alpha Nirvana 39w 8ohm Class A Amp
On the main board the SMT are optional mostly MLCC NP0/C0G caps with silver mica being the through hole equivalent. There are a few that are for snubbers and they are just better when compact. JPS64 will take that into consideration for the final layout. The helper (snubber) PCBs have to be SMT in order to keep the components as close to the legs as possible as they are there to stop oscillation. They are relatively large and easy to solder.
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Last edited by xrk971; Yesterday at 04:24 PM.
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Old Yesterday, 09:19 PM   #174
gary s is online now gary s  Australia
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Thanks for the update Hugh and X. For the 2 circuits as shown, one by Hugh and the other by X, there are some transistor differences. Hugh shows type 2N5401 for Q1, Q2 and Q4, whereas on X's schematic he has these as KSA992. Also Hugh shows 2SC3503 for Q3 and on X's it is a TTC004.

The question is - what are the preferred types for this amp design. Maybe both Hugh and X can give us their views? I think it would be fair to say that Hugh's types are older and the ones X shows are more modern and would be more easy to source (but I have all of them - so what to choose?)

Regards,

Gary..
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Old Yesterday, 10:25 PM   #175
xrk971 is offline xrk971  United States
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You will be ok with either schematic. Hugh’s selection is based more on fact that he has those models handy. I have run the sim with the KSA992, KSC3503, and we may even use BD140 or TTC004A for the Q4. The PCB pinouts will follow the schematic by JPS64.
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Old Yesterday, 11:25 PM   #176
AKSA is offline AKSA  Australia
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Francois G,

Good question. I found after many sims that the loop gain of the 27V rail was 101dB, peaking at 250Hz. For the 20V rail it was 87dB, at 510Hz. In fb amps, any reduction of the intrinsic distortion (the distortion inherent in the topology before any fb), is largely driven by the loop gain, so with a reduction of 14dB for the smaller amp (0.0450% at +20dB) you could reasonably expect distortion around five times more than the larger amp (0.0170% at +20dB).

This is not prescriptive, because other issues intervene, such as component choices, how they behave at lower voltages and to what extent the parasitics influence the dynamic conditions. In this case, we have an improvement of almost 0.03% with the higher power amp, and it is significant that +20dB output for a 20W amp is closer to the rails than for a 39W amp. The best linearity is generally at an output around a third the peak power.

Gary S,

My original schematic is the base document, on which I have done all the careful LTSpice measurements. JPS, the layout designer, and X, the coord/production manager, have readjusted it a little for the production schematic which is identically topologically. All the LTP and VAS devices used, 5401 v. A992 and C1845 v. C3503 and outputs FQA40N25/FQA36P15 v. IRFP240/IRFP9240, will all work just fine. I like to use the 5401 because it is a very tough transistor (600mA/160V v. 50mA/120V) and its 630mW max dissipation makes it difficult to blow up, and it's a bit faster than the A992 (min 100MHz v. min 50MHz). The 5401 does have a lower beta, but it's much more linear than the very high beta A992, being flat as a billiard table from at 0.1mA to 10mA, and it's very quiet with low Cob (6pF). The problem is that the pinouts are different between 5401 and A992, and since X and JP prefer the A992 the layout is revised. The KSC3503 is an outstanding transistor for higher power amps, but the A992 is ideal for the VAS of a low power Class A. I have no difficulty with using any of these devices, but as for every component in any build, you MUST TAKE CARE WITH PINOUTS.

Ciao,

Hugh
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Last edited by AKSA; Yesterday at 11:31 PM.
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Old Today, 04:51 AM   #177
Francois G is online now Francois G  United States
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Quote:
Originally Posted by AKSA View Post
Good question. I found after many sims that the loop gain of the 27V rail was 101dB, peaking at 250Hz. For the 20V rail it was 87dB, at 510Hz. In fb amps, any reduction of the intrinsic distortion (the distortion inherent in the topology before any fb), is largely driven by the loop gain, so with a reduction of 14dB for the smaller amp (0.0450% at +20dB) you could reasonably expect distortion around five times more than the larger amp (0.0170% at +20dB).
Thanks for this great explanation of the differences in expected performance. It will be instructive to see how they behave in physical builds. Thanks again.
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Old Today, 07:42 AM   #178
gary s is online now gary s  Australia
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Thanks Hugh for the detailed response to the transistor differences, and in particular the pin out warning, appreciated.
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Old Today, 09:11 AM   #179
maty tinman is offline maty tinman  Spain
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I think many will be interested.

In Audiocircle forum I wrote:

Quote:
xrk971 detected and measured what happened with a resistance from the new DIY class A ALPHA. He put a thin metal resistor and the harmonic profile changed to H3 predominant. It changed to a carbon and was H2 predominant again. Then changed to a thick metal and kept H2 predominant.
Archimago's Musings: The "Measurement Of Amplifiers Rig" (MOAR): Standard Tests, Loopback results, and the AMOAR Score.

-> Power resistors for amplifier testing – the solution – Clockworks Signal Processing

Click the image to open in full size.

Quote:
With respect to inductance, here’s the measured values:
  • Dale RH 50W 4 ohm resistors are 190 nH
  • Four Reidon 2 ohm 200W in series (8 ohms total) + 4′ of wire 574 nH
  • Memcor 450W 4 ohm wirewound 11 uH
  • Ohmite 300W 8 ohm wirewound 24 uH
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