I'm having a real weird issue with my Altec 1593B. I have two 1593B amps, one sounds great but the other has no bias on the power transistors at all. At very low volumes the sound is distorted, as expected, but as the volume is turned up the transistors are biased and starts to sound better and at loud volume sounds fine, since the bias gets close to the operating point.
I have attached the schematic. Here is what I've done so far:
- both circuit boards (the part on the schematic surrounded by the dotted lines) work on the good amp. But neither board work on the bad amp. So it's definitely not the board.
- with no signal, on the bad amp, the base to ground voltage of Q1 and Q3 are around .5V as expected from the circuit. But the emitter to ground voltage which should be around 30mV is zero.
- I tested all transistors and they are all fine. Still, I replaced all transistors with a new set and still the same. I also replaced the zener CR5 for the 15V line and the diodes CR2 and CR3.
- all collector voltages are fine at 31V, the OPT looks OK and it's probably OK since it plays fine at loud volumes. The stabistor measure 1.3V as expected. The power resistors measure fine and are not open.
- I re-soldered all connections from the circuit board connector and the connections on the power transistors. All grounds are fine.
Don't know what I'm missing here. Any help is appreciated.
not a tech, but going out on a limb here. if you had a signal tracer. you could work your way from the source to the output transisters at low volume till you find the point the distortion comes in.
It's hard to read the print , but it looks like CR1 and it's dropping resistor supplies 1.2V bias through the center tap of the driver transformer. Inspect the resistor / diodes verify you have 1.2V.........GC
It's hard to read the print , but it looks like CR1 and it's dropping resistor supplies 1.2V bias through the center tap of the driver transformer. Inspect the resistor / diodes verify you have 1.2V.........GC
Yes, that was the problem. I had checked the CR1 a few times and the voltage was 1.3V. But when I took it out of the circuit it just had a 0.7V drop. I tied 2 diodes together and got around 1.1V and it worked fine. I need to get the right stabistor. I tried 2 diodes and 1.1V is too low. Tried 3, got 1.5V...too high. Anyone tried a battery based bias?
Common silicon diodes have .6 V drop but, not exactly....try different types to get the required 1.2V
drop .....I would replace the dropping resistor too......
That circuit is interesting in that it's SS with driver and output transformers. How does it sound compared
to complimentary symmetry or quasi complimentary SS amps ????.............GC
Common silicon diodes have .6 V drop but, not exactly....try different types to get the required 1.2V
drop .....I would replace the dropping resistor too......
That circuit is interesting in that it's SS with driver and output transformers. How does it sound compared
to complimentary symmetry or quasi complimentary SS amps ????.............GC
I already did it. Connected 2 diodes back to back and got 1V. Tried 3 and got 1.5V. I need to play around to get something closer.
I haven't set them up in my main system so I can really tell you how they sound. I have an Altec 1003B running the mids from 250 - 7Khz with a phenolic driver and a super tweeter on top. I bi-amp with a couple of huge Bozak Symphonies doing the bass. The huge 1003B horns are very revealing and personally I found out over many variations of amps that it's really not tube vs SS that matters, but the topology. I tried many full and quasi complementary designs in SS and found that quasi sounds nicer but not as revealing as my BGW 250D fully complementary. Those are incredibly clear and revealing. I'm curious about the 1593B because of the combination of transformer inter-stage and OPT transformer. I should have the 1593b amps setup tonight and I'll let you know how it goes.
I have an Altec 1003B running the mids from 250 - 7Khz with a phenolic driver and a super tweeter on top. I bi-amp with a couple of huge Bozak Symphonies doing the bass.
The 1003B are the horns what drivers do you have mounted to the horns..290's ??? IMHO 250 Hz is a little low even for the 290 with the phenolic diaphragm.
The 1003B are the horns what drivers do you have mounted to the horns..290's ??? IMHO 250 Hz is a little low even for the 290 with the phenolic diaphragm.
I have the 730B drivers. I have the spec sheet and they do mention it goes down to usable 150Hz at low level. I tested the response and at 250Hz the SPL is the same as 300 and above. It starts to go down at about 210Hz and disappears at 150Hz, but you can actually listen to the signal at 150Hz. I can setup my active crossover to either 300 or 250 and at 250 I think it sounds slightly more coherent... hard to say, but I like it better. My crossover is a 48 dB/octave so it cuts really sharp at 250Hz. The 1003B with the 730B is pretty flat between 300 (or 250) to 7KHz with the exception of a big 9dB hump at around 1 KHz. The equalization takes care of that. The 730B starts to drop at around 7 Khz where I crossover to a super tweeter with a simple 1st order cap. Gotta try some tri-amping some of these days.
I already did it. Connected 2 diodes back to back and got 1V. Tried 3 and got 1.5V. I need to play around to get something closer.
I haven't set them up in my main system so I can really tell you how they sound. I have an Altec 1003B running the mids from 250 - 7Khz with a phenolic driver and a super tweeter on top. I bi-amp with a couple of huge Bozak Symphonies doing the bass. The huge 1003B horns are very revealing and personally I found out over many variations of amps that it's really not tube vs SS that matters, but the topology. I tried many full and quasi complementary designs in SS and found that quasi sounds nicer but not as revealing as my BGW 250D fully complementary. Those are incredibly clear and revealing. I'm curious about the 1593B because of the combination of transformer inter-stage and OPT transformer. I should have the 1593b amps setup tonight and I'll let you know how it goes.
Elias
I found a nice combination of diodes that gave very close to 1.3V and I was able to bias the output transistors perfectly to 30mV. After that I found out that one of the transistors (actually a Darligton pair) that drives the inter-stage trafo was shot. Replaced and everything was working fine. Biased and balanced fine.
Connected the pair to the 1003B horns and supertweeter top end on my main system and did a whole equalization. How does it sound? Very nice! I hate flowery words to describe an amplifier...like liquid highs, warm mids, etc.. sounds like I'm trying to sell you a high priced piece of speaker cable :-)...so here are my more technical thoughts:
- the 1593B sounds very close to some of my tube amps that are fixed bias. Cathode bias somehow always sounds muddled to me. The 1593B is definitely more detailed and clean than the tubes. Since the 1003B horns are probably only operating at miliwatt levels with peaks into the 1 or 2 watts, clipping is not a consideration with my setup.
- the sounds seems very coherent to me. Not sure, but I think that transformer phase-splitters may be doing a good job. Always wondered if transformers work better as phase-splitters than active components.
- the 1593B also sounds very close to the quasi-complimentary amps that I have. That's expected since they all use the same output transistor. Both quasi- and the 1593B sound slightly less detailed then full-complimentary. On the flip side, full-comp topology does seem a bit harsh sometimes at the top end. I can't be sure but I suspect that slight mis-match between NPN and PNP maybe a bit apparent on very high resolution speakers like big horns. Any comments?
Good to hear the amp is working . do you think the transformers affect the tonal quality ?
Ironic for years and years designers were trying to get the iron out , And here in a SS
design, They are in.............GC
Originally Posted by 604man
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