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ETI 477 resurrected but with problems.
ETI 477 resurrected but with problems.
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Old 9th December 2018, 03:40 AM   #21
Leinster Lad is offline Leinster Lad  Australia
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Quote:
Originally Posted by janusz View Post
They are upgrades in almost any area. VAS in eti477 takes enough current to make transistors without correcting diode/resistor very hot, with diode they are just warm. Nevertheless Sanyos are just better.

cheers,

Thanks Janusz,
So just to be clear.
I can just directly substitute the original BF's with your quoted 2sa1209 and 2sc2911 in the VAS ( Q5, Q6, Q7, Q8 ??) with no other mods ?


You also mentioned an "anti clipping" diode ?
Is this the 18v zener that has been discussed, fitted between the collectors of Q7(BF470) and Q5 (BF469) ?
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Old 10th December 2018, 08:39 AM   #22
janusz is offline janusz  Australia
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Quote:
Originally Posted by Leinster Lad View Post
Thanks Janusz,
So just to be clear.
I can just directly substitute the original BF's with your quoted 2sa1209 and 2sc2911 in the VAS ( Q5, Q6, Q7, Q8 ??) with no other mods ?


You also mentioned an "anti clipping" diode ?
Is this the 18v zener that has been discussed, fitted between the collectors of Q7(BF470) and Q5 (BF469) ?
Yes, you just drop 2sa1209/sc2911 instead of BFs. No need to change anything.

No, the 18V zener diode balances currents and reduces VAS operational temperatures.

Anti-clipping diode is D16 BAS21 in the asc file I have attached. Original eti477 did not have it and it is not essential. I tried it only in simulation.

Simulation also shows improved distortion figures if q5-q6 are ksa992 while q3-q4 and q7-q8 are ksa1142/ksc2682 but simulation results may be misleading as results depend on transistor model parameters and algorithms. Reality is usually at least somewhat different.

Simulation with ksa/ksc transistors gives extremely good results so I do not believe reality would be equally good. At 2kHz and with 8 ohm load and anti-clipping bas21 in thd is 0.000029%. Seems too good to be true.

cheers,
PS put VAS transistors on bigger heatsinks, the original ones are too lousy. Especially if you did not use the 18V zener.

Last edited by janusz; 10th December 2018 at 08:43 AM.
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Old 10th December 2018, 09:27 AM   #23
googlyone is offline googlyone  Australia
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ETI 477 resurrected but with problems.
The caps shown on post 14 were if I recall a mod on the original design to improve stability.
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Old 10th December 2018, 10:42 AM   #24
Ian Finch is offline Ian Finch  Australia
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Yes, that's how the extra greencaps supplied in kits were noted, back in the days of the ETI 5000 (the stereo amplifier that featured the ETI 477 boards). As suggested by others, the sound quality as originally presented, wasn't as good as David Tilbrook's later versions like AEM 6500 which in more recent times, suzyj nicely recreated in SMT for us at diyAudio to marvel at.
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Old 11th December 2018, 12:54 AM   #25
Leinster Lad is offline Leinster Lad  Australia
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Quote:
Originally Posted by janusz View Post
Yes, you just drop 2sa1209/sc2911 instead of BFs.

Anti-clipping diode is D16 BAS21 in the asc file I have attached. Original eti477 did not have it and it is not essential. I tried it only in simulation.

How to open the "asc" file ?
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Old 11th December 2018, 01:02 AM   #26
nigelwright7557 is offline nigelwright7557  United Kingdom
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Some of these old amps can be on the edge of oscillation.
A couple of years back I bought in a Maplin 1980's 225WRMS power amp with some blown and missing parts. I replaced the parts with brand new parts and the amp started oscillating badly. I had to increase the VAS capacitor to stop it oscillating.
I can only guess the new transistors had wider bandwidth or more gain or both hence the problem.
In regards to your amp I would start by replacing all the electrolytics.
I would then set DC offset and bias current.
Connect a load and look for oscillation.
If it oscillates then you will need to change VAS capacitor to a slightly larger value and try again.
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Old 11th December 2018, 01:58 AM   #27
Leinster Lad is offline Leinster Lad  Australia
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Originally Posted by janusz View Post
Yes, you just drop 2sa1209/sc2911 instead of BFs. No need to change anything.

The 2sa1209/2sc2911 seem to quite difficult to get.
None of the big electronics distributors ( RS, mouser, digi key, farnells ) even list them.


fea-bay has some stupidly expensive ones in the UK, and the china ones are of unknown quality, could even be fakes.



Jaycar still have the BF's listed, so might just need to stick with these?
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Old 11th December 2018, 01:59 AM   #28
Leinster Lad is offline Leinster Lad  Australia
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Originally Posted by nigelwright7557 View Post
If it oscillates then you will need to change VAS capacitor to a slightly larger value and try again.

Sorry to have to ask, but which cap is the VAS one ?
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Old 11th December 2018, 04:18 AM   #29
meanie is offline meanie  Singapore
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Think i still kept a similar kit made in HK (Sound Master TA-477)some 20 years ago.
Let me search it up and assemble the kit.
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Old 11th December 2018, 05:17 AM   #30
AKSA is offline AKSA  Australia
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The lag comp cap is always straddling base and collector on the VAS - voltage amplifier - which drives the bias generator of the output stage.

Usually it's a fast resistor, often npn, and chosen to have lowish collector current (up to 200mA rated no higher) and low input capacitance, less than 5pF. This is often a video transistor, getting hard to come by now, originally used in TV sweep circuits that drive the screen.

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