Ben & other techies-Need your opinion

heiney9

Looking at purchasing this amp on flea-bay. The guy has good feedback but based on his description he has just aquired this amp and doesn't know the history. He stated two of the screws from the top plate were missing when he got it. It powers up and both channels work.

My question is looking under the hood the L and R output trannies are different colors. This seems like it could be a red flag to me or this could be normal.

Any input on whether this has been messed with or repaired just by looking at it.

The trannies are supposed to be (10) Toshiba 25c. 3281 & (2) SA 1302



TIA

H9

Lowell_M

That's a pretty beefy looking amp! What is it?

heiney9

a_mattison wrote: »

That's a pretty beefy looking amp! What is it?

Forte 1a- Threshold "budget" company with Nelson Pass as the primary circuit designer. 50 wpc @ 8 pure Class A; 80 wpc @ 4 ohm. Built and designed in the late 80's (88-89) before NP left Threshold.

H9

heiney9

Please feel free to still comment, but it appears this is normal for these amps. Here is a link to a review for the Forte Model 1 the predecessor to the 1a and in the photo you can make out the same color trannies (just fewer of them). So I assume we're all good here.

But further opinion, comments, bashing, etc are always welcomed

http://www.hifi-museum.com/lib/threshold_lib/forte_rev.pdf

H9

Ricardo

We just want the ebay link to bring the price up

heiney9

Ricardo wrote: »

We just want the ebay link to bring the price up

Funny Ricardo , that's why I was keeping it "top" secret. I'm just annoyed because on Saturday there was a listing on Audiogon for a Pass labs Aleph 30 for $650, a killer deal.........I mean killer. By the time I saw it; it had over 175 views. I sent the guy and offer and e-mail and never heard a damn thing either way. I suppose since it was that good of a deal (they usually sell for $900-1100) it was snapped up by the first viewer and guessing by the # of views by Sunday night he probably had a couple hundred e-mails/offers.

The Forte (this particular model) is probably the next best thing. I'm not going to pay a lot for it because I don't know the history but if I win great, I'll be a happy camper.

H9

dorokusai

I thought you hated eBay and rail against it often? The amp looks normal to me.

heiney9

dorokusai wrote: »

I thought you hated eBay and rail against it often? The amp looks normal to me.

Not me, it's not my favorite place to do business. I had the issue with the LSi 7's I bought that never made it to me. The seller was superb in handling the claim with UPS and I got my money back.

I am leary of things on E-bay but I realize the risk/reward consequences and I'm OK with it.

You have me confused with someone else.

H9

billbillw

I'd say that this amp has been worked on. Most likely, something went and took out one whole channel. If the work was done correctly, this may not be a problem. It is possible that the replacement transistors were the same model, but the color had changed over the production run. Really no telling without getting a close look at things.

Unless its a steal, I'd stay away.

heiney9

billbillw wrote: »

I'd say that this amp has been worked on. Most likely, something went and took out one whole channel. If the work was done correctly, this may not be a problem. It is possible that the replacement transistors were the same model, but the color had changed over the production run. Really no telling without getting a close look at things.

Unless its a steal, I'd stay away.

That was my thought until I read the review I posted a link to. In the Audio Magazine review the pic clearly shows two different color trannies (black and green) for each side from the factory. So I'm confident that is normal, but that doesn't mean it hasn't been worked on.

Actually if you study the design it's damn near impossible to kill and there are no adjustments so what could there have been to work on?

H9

dorokusai

heiney9 wrote: »

Not me, it's not my favorite place to do business. I had the issue with the LSi 7's I bought that never made it to me. The seller was superb in handling the claim with UPS and I got my money back.

I am leary of things on E-bay but I realize the risk/reward consequences and I'm OK with it.

You have me confused with someone else.

H9

Roger. I remember that LSi deal....sorry to hear that.

billbillw

Well, if the mis-matched colors are from the factory, then I'd say there is no reason to believe there is any problems with this amp.

ben62670

I have no idea why they used 2 colors. I could see if the NPN was one color, and the PNP was another, but then you would have 2 each per side. Its simple enough to work on if need be, but without doing some test like DC offset I don't know what to tell you.

MikeC78

From that stock pic it clearly shows different color transistors on each channel. The amp on Ebay looks stock to me, I'd go ahead and give it a try. Nice looking amp BTW...

If someone has worked on that amp in the past, I'd say they did a darn good job.

Mike

Face

heiney9 wrote: »

I'm just annoyed because on Saturday there was a listing on Audiogon for a Pass labs Aleph 30 for $650, a killer deal.........I mean killer. By the time I saw it; it had over 175 views.

There are a few Aleph 5 clones on A-gon that appear to be well built. I might pick one up, just to try something different.

Systems

Can't see it in the pic but there should be another row of transistors below the ones you see,so 5 of each type for a total of 10 outputs per chan. PNP's are green,NPN black.

heiney9

GV#27 wrote: »

Can't see it in the pic but there should be another row of transistors below the ones you see,so 5 of each type for a total of 10 outputs per chan. PNP's are green,NPN black.

You are correct; that's how they describe it in the review. 10 per channel parallel in a push/pull config. I am assuming this is a common ground design. Sure looks like it to me.

It does say: "Thanks to totally seprated grounds there is no possible interference between channels"

Systems

heiney9 wrote: »

I am assuming this is a common ground design. Sure looks like it to me.

It does say: "Thanks to totally seprated grounds there is no possible interference between channels"

Yes common ground in that the Neg output is at ground potential.Looking at the pic you can see that the Neg out binding post is connected by a black wire to the star ground on the power supply.However you can also see(bottom center of pic.) another black wire running from each channels power supply ground ,these appear to be connected to the chassis via a resistor, so would isolate them from each other and chassis ground.

billbillw

Ok, I get it.
I was thinking since this is Class A, it was single ended, but further reading revealed it was push pull, biased to Class A.

I should have noticed this earlier. With a push pull, you can't have an un-even amount of transistors; they have to be matched pairs. The five you see on each channel must be part of a pair for each side. The other set on each channel must be down lower on the heatsink and hidden in the shadows of this picture.

By the way, it should have 5x 2SC3281 and 5x 2SA1302 per channel for a total of 10 transistors per channel. That is the NPN/PNP pairing from Toshiba. That document is misprinted. One of those is green, the other is black.

5 matched pair is what you would typically find in a 200wpc class AB amp. Goes to show you how much less efficient Class A is. So much is wasted in heat, but so much smoother sound.

heiney9

billbillw wrote: »

Ok, I get it.
I was thinking since this is Class A, it was single ended, but further reading revealed it was push pull, biased to Class A.

I should have noticed this earlier. With a push pull, you can't have an un-even amount of transistors; they have to be matched pairs. The five you see on each channel must be part of a pair for each side. The other set on each channel must be down lower on the heatsink and hidden in the shadows of this picture.

By the way, it should have 5x 2SC3281 and 5x 2SA1302 per channel for a total of 10 transistors per channel. That is the NPN/PNP pairing from Toshiba. That document is misprinted. One of those is green, the other is black.

5 matched pair is what you would typically find in a 200wpc class AB amp. Goes to show you how much less efficient Class A is. So much is wasted in heat, but so much smoother sound.

You're right billbillw, I always think of class A as single ended as well. I'll keep an eye on this and set my snipe bid accordingly. These don't come up too often and the later Forte Model 4, 4a and 5 were not NP designs and they actually used IGBT's (Insulated Gate Bi-polar trannies)which are not only hard to find now, but not the best choice for outputs.

Thanks to everyone for the input.

H9

heiney9

billbillw wrote: »

By the way, it should have 5x 2SC3281 and 5x 2SA1302 per channel for a total of 10 transistors per channel. That is the NPN/PNP pairing from Toshiba. That document is misprinted. One of those is green, the other is black.

5 matched pair is what you would typically find in a 200wpc class AB amp. Goes to show you how much less efficient Class A is. So much is wasted in heat, but so much smoother sound.

Yes, the Model 3 is basically the same; uses the components in class A/B config and puts out 200 wpc.

H9

Lowell_M

So...Did you buy it yet?

heiney9

a_mattison wrote: »

So...Did you buy it yet?

The bidding ends Sunday. Plus i e-mailed the seller with a few questions about condition and history

Systems

billbillw wrote: »

I was thinking since this is Class A, it was single ended,

But even if it was single ended it would'nt have different devices for each channel.(ie. PNP right ch,NPN for the left ch)


It appears they use the same PC board for each channel but they are installed 180 degrees in relation to each other.

billbillw

GV#27 wrote: »

But even if it was single ended it would'nt have different devices for each channel.(ie. PNP right ch,NPN for the left ch)


It appears they use the same PC board for each channel but they are installed 180 degrees in relation to each other.

Yeah, that's why I thought it had been repaired when I first looked at it. Come to find out, those transistors could be either green or black and it varied through the production run. In otherwords, Green wasn't always the PNP and vice versa.

Systems

billbillw wrote: »

In otherwords, Green wasn't always the PNP and vice versa.

hummm, my understanding was that when green was used in a complimentry pair, it was always to identify the the PNP part,but both could be black.


But yes I see what you mean if they did change them during their production life.

unc2701

Guess I'm a little late on this one, but I can confirm that it's NPN and PNP. They didn't mirror image the boards, so the opposite type is on top on each side. Note that the right side has the thick Orange and red wires on top, whereas the left has the black and blue in the complementary spots (and other components are swapped top to bottom...)

I nearly bought one of these off CL awhile back & thought the same thing when we popped the top... but a little poking around &we figured it out.

Systems

Good luck on your bidding,it should be a good sounding little amp.

heiney9

GV#27 wrote: »

Good luck on your bidding,it should be a good sounding little amp.

I let it slip by. I had too much time to talk myself out of it; now I wish I would have bought it. Oh well.............

H9