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Old 13th July 2011, 12:25 AM   #131
AndrewT is offline AndrewT  Scotland
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JFET input, MOSFET VAS, LATERAL output = Perfect!!
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Old 13th July 2011, 12:28 AM   #132
AKSA is offline AKSA  Australia
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Andrew, any further suggestions? Your earlier one was correct, and I have made the changes, I believe this hits the spot. How did your tests go with and without lag compensation?

I have done a dissipation study of the semis. With an output stage bias of 500mA, VAS current of 12mA and 36V rails, T1 dissipates 77mW, T2 dissipates 420mW, T3 is 120mW and each of the output devices is 18 watts. The heatsink load is thus 36W, and with a Conrad 300x75 0.42C/W sink the rise above ambient is calculated to around 15C assuming all heat passes to the heatsink. On a day of 40C, a 60C heatsink with the amp operating at moderate output is just acceptable, so it should be OK. At all other temperatures, things look peachy.

Cheers,

Hugh
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Last edited by AKSA; 13th July 2011 at 06:55 AM.
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Old 13th July 2011, 10:53 AM   #133
AndrewT is offline AndrewT  Scotland
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I reported last night that I was listening without the miller comp cap.
I have not tried switching it in and out for comparisons yet.
Without M. comp and FM as source it still sounds nice.
I have supply rails +-28.5Vdc to +-29.5Vdc, so -20dB ref max power is ~2.5Vpk or ~2Vac I am still listening still without volume control and without pre-amp.

BTW the amp perforated board is 1.5" by 2.1" (3.5sqinches). That's 2 pairs of stereo boards from 100sq cm of PCB. Adopting smd could allow a few facilities on the same size PCB. Maybe soft start input mute and unmute, DC detect and signal out to relays/FET switches, DC servo.
It really does need a soft input mute for turning off.
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Last edited by AndrewT; 13th July 2011 at 10:56 AM.
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Old 13th July 2011, 11:05 AM   #134
keantoken is offline keantoken  United States
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Vertical standing resistors could definitely make the layout more compact, but it wouldn't look very nice.

- keantoken
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Old 13th July 2011, 11:08 AM   #135
AndrewT is offline AndrewT  Scotland
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I don't have any upstanding resistors. All are laid flat, 600mW, 1%, 50ppm, metal film.

I seem to remember Holco recommended the leads of their high precision resistors must be radiused after they exit the body and not "bent" at right angles nor bent near the body. This recommendation suits usptanding resistors and if the PCB is vertical it also ensures that all resistors are horizontal and thus more evenly heated to take advantage of the precision.
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Last edited by AndrewT; 13th July 2011 at 11:11 AM.
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Old 13th July 2011, 11:14 AM   #136
AKSA is offline AKSA  Australia
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Andrew,

It is self defeating to make this amp too small if you are dissipating quite high power from the output devices, which therefore should be reasonably far apart for best heat distribution.

Soft mute may be desirable, but it is not essential. In practice I'm sure the slight thump at turn-on is of little concern. DC sense could easily be accommodated on a separate board, with a suitable relay - go for it!

The idea is to keep it simple, and easy to build and get going.

Hugh
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Old 13th July 2011, 11:26 AM   #137
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AKSA View Post
Soft mute may be desirable, but it is not essential. In practice I'm sure the slight thump at turn-on is of little concern.
It does not thump at turn on. It appears to play music instantly.
The bigger problem is turn off.
My one plays normally for about 10seconds and by then the rails are so low the tiny signal I am passing to the speakers becomes more and more clipped for about a further minute as the discharge slows down to a crawl.
A FET input mute that brings the signal to ON and then turns it to OFF as a fade would be ideal.
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Old 13th July 2011, 11:32 AM   #138
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AKSA View Post
It is self defeating to make this amp too small if you are dissipating quite high power from the output devices, which therefore should be reasonably far apart for best heat distribution.
at the moment I am at +-29Vdc and 300mA quiescent current. That's a total of 17.4W. I am using a 15mm thick backplate on a 0.6C/W sink. The devices are direct coupled to the sink (no Source resistors) and yet it runs @ ~ blood heat. There is no problem. My devices are @ the corners of the 1.5" wide dimension, i.e. 5 empty 0.1" pin pitches apart.

If I were designing for Ta<=40degC rather than <=30degC, then I might increase the distance between the PCB and the sink, but I would not consider making the PCB bigger. The gate stoppers become the links between the PCB and the outputs.
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Last edited by AndrewT; 13th July 2011 at 11:42 AM.
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Old 13th July 2011, 10:45 PM   #139
Jozua is offline Jozua  South Africa
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Andrew

I am keen to know how the amp sounds with a decent signal ?

Hugh

Are you ordering spare boards and parts ?



Regards

Jozua
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Old 13th July 2011, 10:51 PM   #140
woodturner-fran is offline woodturner-fran  Ireland
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I have a nice whopping big transformer with dual 37VAC secondaries sitting on my shelf. Too high for this amp?

Fran
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