Switch on back could be a half Arsed hard bias adjustment of some sort , but not a trimmer.
measure the grey resistors ... any of them.
Is the amp working ??
PS - stop using photobucket - post direct to here.
measure the grey resistors ... any of them.
Is the amp working ??
PS - stop using photobucket - post direct to here.
Got the amp today, sound ok I guess, very much like my old B&K ST-202 but with more bass
very well made, heavy construction in the chassis
I saw a few capacitors , tried the back switch 100/200 watt, did not notice any differences, the switch in connected straight to the power supply
A lot of power, maybe way too much for my system
very well made, heavy construction in the chassis
I saw a few capacitors , tried the back switch 100/200 watt, did not notice any differences, the switch in connected straight to the power supply
A lot of power, maybe way too much for my system
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You should be able to improve the sound by replacing those dark brown capacitors with polystyrene and polypropylene film caps. Polystirens are really good up to a few nanofarads, and polypropylene film onwards... The original caps might be silvered mica caps - terrible in amplifiers... especially in AB designs with a fair amount of negative feedback. But, they might be mylar - still bad, but not as bad as silvered mica. Thanks for sharing the photos.
Do you see potential in the amp ?
what does the switch in the back do ?
No bias adjustment either ?
what does the switch in the back do ?
No bias adjustment either ?
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I have a 2300 model that I have to use as a spare. My advice would be to leave it alone if you don’t have tinkering skills.
The switch is a thermal cutout (you should be able to read the temp cutout rating on the switch.... maybe 65 deg C?)
The bias trim pot seems to be replaced by a resistor.... it shouldn't be difficult at all to replace the resistor with a trim pot.
I'd rip the guts out and put a nice 2-gain-stage, single-ended class A PCBs inside... of course, less power, but beautiful sound.. 🙂
The bias trim pot seems to be replaced by a resistor.... it shouldn't be difficult at all to replace the resistor with a trim pot.
I'd rip the guts out and put a nice 2-gain-stage, single-ended class A PCBs inside... of course, less power, but beautiful sound.. 🙂
I see the drivers and vbe in Image1512 , so it is EF2 BJT. if you could identify the 2 resistors on the vbe you could add a trimmer.
Does it work ??
Class A ??? Throw 2 150W Badgers in there and ROCK OUT !!
Does it work ??
Why ?? they last the longest !
Class A ??? Throw 2 150W Badgers in there and ROCK OUT !!
The amp is working fine, it sounds like a hafler xl-280
Should I just recapped or aim for diy audio board ?
Should I just recapped or aim for diy audio board ?
Haha.. then keeping the silvered mica caps is a must... for that beautiful sound that rocks out.
Yes , I reverse engineered your amp by looking at the board. It's design is basically what I posted. It could be "tweaked" with offset and bias trimmers.
It does not have regulated input LTP's , cascodes , "ripple eaters" or a fancy VAS. It could be an EF3.
Three things the improved version of this design does.
- 1. PSRR can be -100db+ , running off a single supply still seems like dual mono.
- 2. .002% instead of .01% 20Khz , this might not be noticeable depending on the application.
- 3. "Fancy VAS" + servo makes the circuit nearly "bombproof".
OS
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I've seen 70 year old silver mica's that still are in spec.
according to the owner the amp was in storage for 20 years, it does look in mint condition
My idea was to send it to my regular tech for a recap with quality cap and better film caps
Do you have any more mods ?
I dont need more power with this amp
My idea was to send it to my regular tech for a recap with quality cap and better film caps
Do you have any more mods ?
I dont need more power with this amp
Cascodes (Q3/4/11/12) are regulated at +/- 12V. Servo (U1) also uses the 12V supplies. Q13-16 is a self clamping Hawksford VAS , can
clip all day and not blow up , LEDs D11-14 absorb the saturation current at clipping. Red led's are thermally negative to the semi's , temperature
have almost no effect on the circuit.
U1 shifts (VCCS1/2) with voltage to balance Q1-2 and Q3-4 , NO offset.
Output stage is the same as my 40 year old Harmon Kardon 680 amp with newer devices and a few tweaks.
Q101/102 are cap multipliers and make a 22uF capacitor seem like a 3000uf cap. Very clean power.
1/10th the THD (.002%/20Khz) of your amp and 30+ db more ripple rejection (-114db) than your amp. a few more parts , but worth it.
It self zero's the offset AND has a refined bias circuit WITH a trimmer to adjust it.
PS - it actually is a old design , dozens were built 8 years ago.
OS
clip all day and not blow up , LEDs D11-14 absorb the saturation current at clipping. Red led's are thermally negative to the semi's , temperature
have almost no effect on the circuit.
U1 shifts (VCCS1/2) with voltage to balance Q1-2 and Q3-4 , NO offset.
Output stage is the same as my 40 year old Harmon Kardon 680 amp with newer devices and a few tweaks.
Q101/102 are cap multipliers and make a 22uF capacitor seem like a 3000uf cap. Very clean power.
1/10th the THD (.002%/20Khz) of your amp and 30+ db more ripple rejection (-114db) than your amp. a few more parts , but worth it.
It self zero's the offset AND has a refined bias circuit WITH a trimmer to adjust it.
PS - it actually is a old design , dozens were built 8 years ago.
OS
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Shrot in spekare wiring?
Speaker wiring ends touching the case?
Oscillations (use the resistor load and oscilloscope to check... and then add a bit of capacitance...)
Are the speakers (binding posts, crossover...) okay?
Speaker wiring ends touching the case?
Oscillations (use the resistor load and oscilloscope to check... and then add a bit of capacitance...)
Are the speakers (binding posts, crossover...) okay?
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