I have been listening to my Forte 1A a lot lately and have been loving it. However, what good piece of equipment couldn't be made better with a few updates. 
Here are my ideas and please let me know what you think.
-Add a toggle switch and a pair of Jensen Transformer JT-10KB-D to create a balanced set of inputs.
-I also think that it would benefit from a cap upgrade and maybe other internals because of age. Nelson seems to be suggesting that upgrading the C2 may do it.
Thanks for the help anyone and I'll try to keep my carpet from catching on fire when I use it to drive my 84db sensitive speakers.
Here are my ideas and please let me know what you think.-Add a toggle switch and a pair of Jensen Transformer JT-10KB-D to create a balanced set of inputs.
-I also think that it would benefit from a cap upgrade and maybe other internals because of age. Nelson seems to be suggesting that upgrading the C2 may do it.
Thanks for the help anyone and I'll try to keep my carpet from catching on fire when I use it to drive my 84db sensitive speakers.
C2 is in series with the feedback loop.
The existance of which just means no gain at DC correct?Otherwise its directly coupled?
I thought the constant current source into a class A output was the aleph patent?
Even if push-pull is in a H system (surround by a H system configuration), it still is not a real Pure Class A. A Pure Class A as nothing to do with push-pull. Most of them are A A/B, but they are not Pure Class A. Maybe that Pure Class A don't exist. Maybe with tube. I think it is easier with tube and they are not very powerfull of course. If you read some electronic basic book, Class A is H configuration, and that all. About Single end (SE), my knowledge is not so extended. I will try to take a look in books or Internet.
That's my point of vu. Sorry, i usually write in french.
Tancrède
That's my point of vu. Sorry, i usually write in french.
Tancrède
Just wondering if Mr Pass still hangs around these premises... I've replaced the caps of my Forté 1a after 25 years of continuous operation for 80V/33mF Nippon Chemi-Con ones I got cheap from a liquidation sale. I don't have the service manual.
So I'm wondering where to measure and what to set for bias. I really don't wanna shoot this unit. Imho nothing comes close in bang-for-buck terms and few are better at *any* price!
So I'm wondering where to measure and what to set for bias. I really don't wanna shoot this unit. Imho nothing comes close in bang-for-buck terms and few are better at *any* price!
Looking at the schematic in post #16 the only pot appears to be R15, and it seems to be something that would increase the bias.
A different post talks abut measuring .46V across the emitter resistors (R22-31) and having a heatsink temp of ~40C. EDIT - But that's a 1, and you say a 1a, and I think one is bipolar, one is Mosfet... So a good place to start would be measuring the DC voltage drop across the Source (or emitter) resistors. They should be big ones, near the output devices.
Papa suggested a max heatsink temp of ~45C.
That's all I have for thread analysis - hopefully somebody who actually knows something will chime in.
A different post talks abut measuring .46V across the emitter resistors (R22-31) and having a heatsink temp of ~40C. EDIT - But that's a 1, and you say a 1a, and I think one is bipolar, one is Mosfet... So a good place to start would be measuring the DC voltage drop across the Source (or emitter) resistors. They should be big ones, near the output devices.
Papa suggested a max heatsink temp of ~45C.
That's all I have for thread analysis - hopefully somebody who actually knows something will chime in.
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I suggest you pick a few emitter resistors and measure the voltage across
them and take notes before you modify the amp.
With a little luck it will still have those values when you fire it back up, if you
don't move the pot.
From there, you can adjust the bias up or down (gently and slowly, allowing
an hour for it to settle) and set it where you can just put your hand on the
top of the sinks for 10 seconds or so.
them and take notes before you modify the amp.
With a little luck it will still have those values when you fire it back up, if you
don't move the pot.
From there, you can adjust the bias up or down (gently and slowly, allowing
an hour for it to settle) and set it where you can just put your hand on the
top of the sinks for 10 seconds or so.
HAHAHA yes, very practical!
Mr. Pass says "increase the bias until the heatsinks run at about 45° C.."
No voltmeter needed! I love it.
You can buy a non-contact (I.R.) thermometer for about $10 these days.... and really you do want as much bias as practical so the amp will stay in class A for lower impedance loads, etc.
In another thread HERE the fellow says that measuring a voltage drop across one of the emitter resistors of around 120 mV is what you want.
I have two Forte 4a amps with the Soderburg mods, I LOVE them. Mr. Pass says he doesn't like the 4a with it's IGBT's all that much, but to me the 4a sounds very good.
I just now bought a 1a, $500 with shipping. Looking forward to it.
Mr. Pass says "increase the bias until the heatsinks run at about 45° C.."
No voltmeter needed! I love it.
You can buy a non-contact (I.R.) thermometer for about $10 these days.... and really you do want as much bias as practical so the amp will stay in class A for lower impedance loads, etc.
In another thread HERE the fellow says that measuring a voltage drop across one of the emitter resistors of around 120 mV is what you want.
I have two Forte 4a amps with the Soderburg mods, I LOVE them. Mr. Pass says he doesn't like the 4a with it's IGBT's all that much, but to me the 4a sounds very good.
I just now bought a 1a, $500 with shipping. Looking forward to it.
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Does anyone happen to know the value of the bias resistors in the Forte 1a? Mine have gotten pretty ragged and I have to keep twisting the wiper on one of the channels to get it to bias up again. I want to get replacements, I suppose I could pull one and measure it but if anyone happens to know the value I'd appreciate it. Thanks!