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Old 20th September 2011, 11:50 AM   #111
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Quote:
Originally Posted by dadod View Post
After readig "The F-word or, why there is no such thing as to much feedback" by Bruno Putzeys http://www.linearaudio.net/userfiles...olume_1_BP.pdf I decided to lower VAS gain, in case if there is enough gain at 20kHz, in my next project too.
I'm nonplussed. So an article which essentially says "use as much loop gain as you possibly can" is now taken as an excuse to lower loop gain? I mean, yes it can be an easy psychoacoustic fix when you've thrown in the towel as far as loop gain at 20kHz is concerned. But what I'm really trying to say is: work on your circuit to make loop gain at 20kHz higher. Use a T network, for instance. It's the quickest way to boost loop gain on a standard circuit.

@JCX: 100% on the mark with the nonlinear C. There are several "highly regarded" amps out there with constant (or no) loop gain over the audio range where this distortion mechanism dominates starting from a few kHz. Ouch.
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Last edited by Bruno Putzeys; 20th September 2011 at 11:57 AM.
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Old 20th September 2011, 12:41 PM   #112
dadod is offline dadod  Croatia
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Quote:
Originally Posted by dadod View Post
"I surmise that, since the amplifierís distortion was never negligible, making
it constant across the audio band makes it fly under the psychoacoustic radar more easily."

That is what Putzeys says and probably I took it wrongly. In that exsample the amp distortion was never negligible. But in the case of my amp, in my opinion, distortion was negligible without gain redution at low frequences and stayed negligible with the gain reduction. A bonus is better phase behavior, I was afraid a bit of instabillity.
At Loop Gain diagram you can see that there is more then 60dB of negative feedback.
dado
Bruno Putzeys wrote:
" I mean, yes it can be an easy psychoacoustic fix when you've thrown in the towel as far as loop gain at 20kHz is concerned. "

I did not throw in the towel as there is more then 60dB of negative feedback at 20kHz, I was thinking that this is enough for 20Hz as well. In this case the phase behavior is better and I think i could get more stable amp with enough low distortion( in ppm region).
I used T network called here TCM.
I am open on all sugestions and if this use of the TCM will hart this amp i will drop it and use TCM witout VAS(Mr. Putzeys call it Transimpedance stage and that is correct, but VAS is so dipp in the teminology) local negative feedback.
I will appreciate if Mr. Putheys take a look through out this thread and tell me if my diagrams are wrong.
dado
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Old 20th September 2011, 01:07 PM   #113
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Quote:
Originally Posted by dadod View Post
Bruno Putzeys wrote:
" I mean, yes it can be an easy psychoacoustic fix when you've thrown in the towel as far as loop gain at 20kHz is concerned. "

I did not throw in the towel as there is more then 60dB of negative feedback at 20kHz
Oh in that case I wouldn't bother with things like flattening loop gain. I merely noticed that it was perhaps the least "scientific" bit that was lifted from the F article.
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Old 20th September 2011, 02:50 PM   #114
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Bruno Putzeys View Post
Oh in that case I wouldn't bother with things like flattening loop gain. I merely noticed that it was perhaps the least "scientific" bit that was lifted from the F article.
Here on this forum you can find a lot non "scientific" explanation. I took that part from from the F article as here this amp was made before I have found yor article and what you said looked resonable to me. I suppose you meant it for an amplifier whit not so low distortion.
Main reason I used it, was better phase behavior then whitout it. Distortion was quite low already.
Here are two Loop Gain diagram, one without local VAS FB and one with it.
First one shows phase drop to -174 degree at 20kHz and second one only -154 degree at 80kHz. Phase margin is the same, 65 degree at 2MHz, where the gain drop to zero.
Anyhow I tried both way and I could not hear diference, just I expext the second one could be more stable.
Dado
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Old 20th September 2011, 03:03 PM   #115
dadod is offline dadod  Croatia
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My suggestion for the power supply is here.
dado
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Old 20th September 2011, 03:14 PM   #116
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Hi Dado,

I checked your BOM with my list based on your previous schema. Thatís OK for almost all components (I saw some changes in capacitor and resistor values).


Iíve some questions :

a) In your previous schema, Q11 was BC556B and Q12 BC546B. In your last schema, values are inverted. What is correct ?

b) D01 and D02 are MUR460 in your previous schema. I found ON SEMICONDUCTOR MUR460G. Are other types possible ?

c) Reference for L1 (2uH) ?

d) Q1, Q2, Q3 (2SA970). I found Toshiba 2SA 970BL. OK ?

e) Q5, Q6, Q7 (2SC2240). I found Toshiba 2SC 2240BL. OK ?

f) Q8, Q96 (NJW4281). I found ON SEMICONDUCTOR NJW0281G. OK ?

g) Q10, Q97 (NJW4302). found ON SEMICONDUCTOR NJW0302G. OK ?

h) Q9. I found Fairchild 2SC3503 STU Series with various hFE. Which hFE to choose (40, 60, 100 or 160 ) ?

i) R83 (1K). Is multiturn trimmer and which power handling ?

j) I found that electrolytics C5, C7 and C10 were normal ESR and other C6, C12, C16 and C17 were low ESR. Is that correct ?

k) Can I replace fuses with a special card for DC detection at speaker output, or are this two types of protection complementary ?

l) Are all resistors 0.25W (but R99 3W) ?


Could explain what is the topology difference with JLH 80W amp in simple words ?


Nicola
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Old 20th September 2011, 04:12 PM   #117
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Hi Nikola,
Here it is:
a) In your previous schema, Q11 was BC556B and Q12 BC546B. In your last schema, values are inverted. What is correct ?

Q11 - BC546B and Q12 - BC556B I did a new drawing with expresSCH software and made a mistake

b) D01 and D02 are MUR460 in your previous schema. I found ON SEMICONDUCTOR MUR460G. Are other types possible ?

Any diode 300-600V and 3A or more.

c) Reference for L1 (2uH) ?

I used 1.2mm diametar enameled wire and coild it on 7mm dia pencil cca 20 coils.

d) Q1, Q2, Q3 (2SA970). I found Toshiba 2SA 970BL. OK ?

OK

e) Q5, Q6, Q7 (2SC2240). I found Toshiba 2SC 2240BL. OK ?

OK

f) Q8, Q96 (NJW4281). I found ON SEMICONDUCTOR NJW0281G. OK ?

OK

g) Q10, Q97 (NJW4302). found ON SEMICONDUCTOR NJW0302G. OK ?

OK

h) Q9. I found Fairchild 2SC3503 STU Series with various hFE. Which hFE to choose (40, 60, 100 or 160 ) ?

I used series E 100-200, F is good to 160-320

i) R83 (1K). Is multiturn trimmer and which power handling ?

Look PCB, it vertical type normal one(very low power handling)

j) I found that electrolytics C5, C7 and C10 were normal ESR and other C6, C12, C16 and C17 were low ESR. Is that correct ?

Only C5 is not important, all others better to have low ESR. C10 is not elko (0.22uF)

k) Can I replace fuses with a special card for DC detection at speaker output, or are this two types of protection complementary ?

I never use a relay in the audio chain. Off course you can use special card, but if it uses a relay to switch of the loudspeaker
conntact will deteriorate durin the time and I don't like that. You can live fuses it will not have any influence.

l) Are all resistors 0.25W (but R99 3W) ?

R98 should be 2W and R99 3W, all others 0.25W is OK.

Regarding JLH 80W amp, it uses lateral output FETs and uses quite low negative feed back, cca 20dB at 20kHz.
My modification of JLH uses cca 40dB of NFB at 20kHz.
I am still listening this amp, and it is very stable, not so before mdification.

This amp here is classic one with all bipolar transistors, but with very high negative feedback cca 60dB at 20kHz. It is possible to have it so high because it uses different compensatio not standard Miller compensation, so called TMC. If you like to know more about it search this forum, there it was a lot discution about it.
Dado
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Old 20th September 2011, 05:29 PM   #118
vac231 is offline vac231  Switzerland
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Oups, I forgot to ask for the rated voltage of the electrolytics. 50V is for the audio capacitors (silmic). But for others could I use lower tension (more easy to find) ?
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Old 20th September 2011, 08:52 PM   #119
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C3 could be 10V and if you can find with so low voltage you can increase the value even up to 1000uF. D. Self recomandet here high value but best quality you can find, Elna silmic or better.
C5 could be less then 50V but not less then 30V if there is a one with that voltage.
C7 better to be 63V.
all others 50V is good enough.
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Old 21st September 2011, 01:56 AM   #120
vac231 is offline vac231  Switzerland
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Hi Dado,

A. About other capacitors not electrolytics. Is the quality not significative for some of them (like the green C4) for example ?

B. I see the central thermal aluminium you use to cold Q94 and Q95 :

B1. Is this main dissipator DIY or available somewhere ?

B2. You have added an additional piece of metal to it.. Are Q94 and Q95 so hot ?


Have a nice day!

Nicola
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