Modifying Audiolab 8000A

[RichLund]

Hi

I have an old ('B' series) 8000A amp that I would like to upgrade as a project. I plan to upgrade the following items:

1. Replace RCA socket 'blocks' with gold plated panel mount sockets hard wired with silver plated hookup wire.

2. Replace naff speaker terminals with higher quality gold plated variety.

3. Replace Elna 10000uf 50V power filter caps with Nichicon Gold Tune 15000uf 63V caps

4. Replace 4 x 1n4002 rectifier diodes with fast recovery uf4002 equivalents.

And finally I was considering replacing most or all caps in preamp section with black gates or Elna silmics. I am having trouble sourcing these caps with the required voltages. Is it ok to use caps with a higher rated voltage. I.e: replacing a 50V cap with a 100V equivalent. What would the risks be?

Has anybody modified the 8000A before. What component changes would you recommend to further improve the sound?

Thanks in advance for advice

Rich

[sreten]

Hi,

Higher voltage ratings are of course perfectly fine, in fact a type
of capacitor distortion decreases the higher the voltage rating.

/sreten.

[RichLund]

Excellent news!

Cheers sreten.

[carlmart]

Originally posted by RichLund

Has anybody modified the 8000A before. What component changes would you recommend to further improve the sound?

I used to have a service manual for this amp, but not anymore. Can you put the schematic here, so we can suggest improvements?

[brainf]

Originally posted by carlmart


I used to have a service manual for this amp, but not anymore. Can you put the schematic here, so we can suggest improvements?

Would this be the one?

[carlmart]

Originally posted by brainf


Would this be the one?

Yeah, that's it. Now I remember why I threw it out.

Several things I didn't like on it:

1) Zener regulators for the first stages.

2) No filtering tor the zeners and none after the 22 ohm resistors.

3) Not the best IC for the servo and it was a dual one, so not separate for each channel.

4) The collectors on the output transistors should be directly fed to the supplies and not through that layout.

OTOS, didn't this power amp use a regulator for the high current sections too? I think I remember something like that, which would invalidate some of my critics on filtering.

[AndrewT]

Originally posted by RichLund
....Has anybody modified the 8000A before....?

Hi Rich,
you clicked "I have searched" but didn't.

[RichLund]

Thanks for the reply's guys

Andrew your input is definitely welcome - I did do a search but the main theme of the threads seem to be repairing faults which, to be fair I don't have. This is simply an upgrade as a project (a slightly expensive one ) to see if i can squeeze a better more detailed / fuller sound out of the amp. So I am just following basic priciples such as upgrade caps and diodes to the best i can afford. I know this amp has severe criticisms but I do love the 'ol bugger.

I was actually wondering if there is a way to bypass the balance pot, things like that.

Carlmart - you mentioned the zener regulators, what would you suggest to improve here. Obviously better regulators but what type.

I used to work in an electronics factory assembling and testing amplifier PCB's etc. ( we made the first generation of Videologic Sirocco and Crossfire amp/speaker systems for PC's. Anybody remember these?) I am not trained in electronics but i know my way round a PCB.

My blackgates arrived today and as I write am in the process of replacing the Elna's in the preamp section.

After some correspondence with Brent at Audio Upgrades I have ordered some HexFred - Ultra-Fast, Soft Recovery Diodes - HFA04TB60 for the power section. So far so good, fingers crossed!

As a note, the circuit schematic above is slightly different to my PCB, such as only two 1N4002's - mine has four!

If this works out, then I suppose this would be a worthy upgrade method for this amp. Thanks again for the responses, keep them coming.

[carlmart]

you mentioned the zener regulators, what would you suggest to improve here. Obviously better regulators but what type.

Chip regulators, particularly those in the 3X7 families, tend to sound a lot better than zeners.

Improving the servo chip, with an LF412 perhaps, might improve things too.

Of course you are planning to improve on the small value caps, right? That is more important than electrolytic caps usually. Eliminate any ceramics you may have and put polystyrene to replace them. And polypropylene for the higher values, particularly at the input.

Going higher on the main capacitors might improve things a bit, but you are already dealing with that. Don't spend too much money on Hexfreds. You may find Motorola fast/low types that are as good or better and cost much less.

Do some reading here:

http://www.tnt-audio.com/clinica/ssps1_e.html

Pay attention to the split bridges he used for each channel and the RC filters at the output.

[RichLund]

Carlmart my man, that was a superb reply. Getting to the nitty gritty here..

ok, please excuse my ignorance in asking the following:

Replacing the regulator zeners with chips (such as LM317T?) doesn't seem easy as the chips have three pins. The voltage in and out pins seem obvious as long as its orientated in the same direction as the zener but is the centre pin (ADJ) bridged to the output resistors? I hope i have asked correctly. I have researched on google but just want to make sure from an expert.

I have sourced An LF412 on ebay, but with varying costs. What is the difference between a LF412ACN / ACH / CN etc. Which do i need?

with regards to ceramic caps, there seems to be only three on the board and they are in the phono stage which i'm leaving alone. The board seems to have an abundance of polypropylene caps (square blue plastic packages?) but i will reference with RS or Farnell to make sure.

That article you linked to was spot on, many thanks.... learning, learning, learning.

Thanks again bud

Rich