The rest of the story
You guys will probably find the rest of this tale somewhat amusing.
I finally acquired a pair of 15335A transformers on eBay for my 9440A and they work fine, just like you said they would. I never got around to cancelling my eBay alert emails for the 15095A, however, and I saw one about to close for well below what they'd been going for. Not expecting to win it, I sniped it for a few bucks over the opening bid and it closed without a second bid.
Since the diagrams for both transformers show pins 7 and 8 as being the 15K-ohm secondary, I expected that all I'd need to do is jumper pins 3 and 4 to configure the primary for 600 ohms, but when I looked at the 9440A schematic, I discovered that the sockets were wired so that pins 7 and 8 were used as the primary. Not to be deterred, I made an adapter to correct the wiring and tried it out. Using only a microphone as a source, I found the voltage gain to be just under 10 dB more than with a 15335A installed. Of course, one doesn't generally drive a power amplifier directly from a microphone, but I had to test the transformer and I knew there was going to be enough gain to do the job. Now my curiosity is satisfied and the 15335As are back in the sockets.
You guys will probably find the rest of this tale somewhat amusing.
I finally acquired a pair of 15335A transformers on eBay for my 9440A and they work fine, just like you said they would. I never got around to cancelling my eBay alert emails for the 15095A, however, and I saw one about to close for well below what they'd been going for. Not expecting to win it, I sniped it for a few bucks over the opening bid and it closed without a second bid.
Since the diagrams for both transformers show pins 7 and 8 as being the 15K-ohm secondary, I expected that all I'd need to do is jumper pins 3 and 4 to configure the primary for 600 ohms, but when I looked at the 9440A schematic, I discovered that the sockets were wired so that pins 7 and 8 were used as the primary. Not to be deterred, I made an adapter to correct the wiring and tried it out. Using only a microphone as a source, I found the voltage gain to be just under 10 dB more than with a 15335A installed. Of course, one doesn't generally drive a power amplifier directly from a microphone, but I had to test the transformer and I knew there was going to be enough gain to do the job. Now my curiosity is satisfied and the 15335As are back in the sockets.
honestly do not know what else to say thank you very much I also have a broken 944a without knowing where to find the schema? Fortunately there are those who wholeheartedly shared enthusiasm as you!
Hi, mind if I hijack this thread with an amplifier-design-ish question? The Altec 9440a, according to the schematic I have and what I see in the unit on my bench, has 4.7 ohm resistors paralleling the output transistor bases. Thus to get a Vbe high enough for the outputs to have a bit of idle bias current - say, 600mV Vbe - will mean a current of about 130mA through those resistors.
But the bias procedure in the manual calls for a total of 75mA into the driver/output board as a whole, so the 4.7 ohmers could develop no more than about 350mV. That can't be enough to turn on the outputs. It certainly is not, with the outputs I have installed (2N6259).
So what am I missing? The original outputs WERE silicon, weren't they? Did the Altec designers intend that the outputs not have any idle current?
Thanks,
chazix
But the bias procedure in the manual calls for a total of 75mA into the driver/output board as a whole, so the 4.7 ohmers could develop no more than about 350mV. That can't be enough to turn on the outputs. It certainly is not, with the outputs I have installed (2N6259).
So what am I missing? The original outputs WERE silicon, weren't they? Did the Altec designers intend that the outputs not have any idle current?
Thanks,
chazix
Thanks. I guess I was hoping for some magic alternative answer, though.
My example is showing some oscillation when negative-going outputs get to the point where the common-emitter half of the output stage starts to conduct. I'm concerned that the output devices I installed are "too fast" even though they claim to be the same JEDEC number.
I'm half-thinking about bumping the parallel base resistor from 4.7 to 10 ohms, and adjusting bias to obtain some idle current in the outputs, so that there's not such a gross transfer function discontinuity.
Cheers,
chazix
My example is showing some oscillation when negative-going outputs get to the point where the common-emitter half of the output stage starts to conduct. I'm concerned that the output devices I installed are "too fast" even though they claim to be the same JEDEC number.
I'm half-thinking about bumping the parallel base resistor from 4.7 to 10 ohms, and adjusting bias to obtain some idle current in the outputs, so that there's not such a gross transfer function discontinuity.
Cheers,
chazix
Crown uses an LCR combo from the emitter of the predriver to the output rail.
http://www.crownaudio.com/media/pdf/legacy/137557-3_B_sch.pdf
The D60 uses a simplified version.
http://www.crownaudio.com/media/pdf/legacy/d60-im.pdf
http://www.crownaudio.com/media/pdf/legacy/137557-3_B_sch.pdf
The D60 uses a simplified version.
http://www.crownaudio.com/media/pdf/legacy/d60-im.pdf
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So I went off to do an ad hoc experiment along the lines of djk's tip (thanks for that!), and found that the Altec driver boards have un-populated (in my example) locations for several inductors, L1 thru L4. For each of the pre-driver transistors, these absent inductors would parallel the emitter resistor and the collector R+C+diode network.
Does anyone happen to have an example where L1-L4 are installed, or a schematic that shows them? If so, can you identify their value, and also the values for the Rs & C at the emitter and collector of the predrivers?
Thanks!
chazix
P.S. The ad hoc experiment (read "kludge") I did was just to insert an inductor in series with the existing pre-driver emitter resistor. Didn't kill the oscillation, but it lowered in frequency from around 4MHz to around 1MHz
Does anyone happen to have an example where L1-L4 are installed, or a schematic that shows them? If so, can you identify their value, and also the values for the Rs & C at the emitter and collector of the predrivers?
Thanks!
chazix
P.S. The ad hoc experiment (read "kludge") I did was just to insert an inductor in series with the existing pre-driver emitter resistor. Didn't kill the oscillation, but it lowered in frequency from around 4MHz to around 1MHz
Haven't dug up any info on what Altec had in mind with the un-populated places for inductors, so I'm thinking of taking the easy road, and just adding Miller capacitors to the pre-drivers. 100pf seems to be just enough to quench the oscillation, so I'm thinking of using 150pf.
How dumb is this idea?
chazix
How dumb is this idea?
chazix
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