Krohn-Hite Model 7500 Amplifier - diyAudio
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Old 5th October 2006, 02:55 AM   #1
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Default Krohn-Hite Model 7500 Amplifier

I found this potential boat anchor at Boeing Surplus for $35.

Yeah, I know, it's an instrumentation amplifier; swings a heck of a lot of voltage but not very much current. Internal construction was strange, as I'd expected: lots of driver-type transistors in the middle-back of the chassis, probably a series of amplifier modules in parallel. I'm half-wondering if it'd be at its best driving electrostatics? Date codes are mostly 1976-77, but it's pretty clean and nothing looks toasty.

The writeup suggests the amplifier works, but isn't meeting specs and someone at Boeing decided to toss it. If nothing else, it'll make a really fine 4.1 chassis.

Anyone got a schematic for this beastie? Might be interesting reading.

I sometimes take in lost causes like this; I've also got a Crown IMA intermodulation distortion analyzer that was definitely busted--both meter movements are shot. Got it for $30 and might someday take a crack at getting it to work again.
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Old 5th October 2006, 04:45 AM   #2
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"I'm half-wondering if it'd be at its best driving electrostatics?"

With 140Vrms??

"Anyone got a schematic for this beastie"

Krohn-Hite will sell it for 150$!

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Heinz!
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Old 5th October 2006, 08:54 AM   #3
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$150 is a bit dear for a schematic of a 30 year old amplifier design.

Loath as I may be to vandalize this otherwise fascinating piece of laboratory equipment, the chassis looks very usable for an audio amplifier project.
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Old 5th October 2006, 04:11 PM   #4
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"Loath as I may be to vandalize this otherwise fascinating piece of laboratory equipment, the chassis looks very usable for an audio amplifier project."

A sacrilege!!
This amp is still in production and costs some thousends $!
Test it, when the specs are still nice put it to ebay, I am shure You will make a good deal!

So far I know first in 1990 this device get competitors in specs
from a japanese and my company!

I bid 150$ plus post and packing and insurance (another 150$ ?)

Regards
Heinz!
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Old 5th October 2006, 06:09 PM   #5
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Your offer is tempting.

Yeah, I hate to tear up a clean piece of gear, but I'm equally reluctant to let it go before I have a chance to >understand< this design. Defiitely I'll attempt to test it into a suitable impedance load (somewhere around here I have a 100 ohm power resistor).

Main power supply capacitors are rated at 350 volts! Unusual to see in solid-state equipment, especially from the 70s. Krohn-Hite used to offer a similar type of amplifier, but with tubes and optional matching transformer for lower impedance loads.
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Old 6th October 2006, 10:30 PM   #6
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"Main power supply capacitors are rated at 350 volts! Unusual to see in solid-state equipment, especially from the 70s. "

Indeed, because the output can deliver +/- 200V!
Can You indentify the output transistors?

"Krohn-Hite used to offer a similar type of amplifier, but with tubes and optional matching transformer for lower impedance loads."

?? As far I know the model 7500 is throughout solid state and I don´t see ever a tube amp from K-H!
The matching transformer is a additional part for the 7500.

Regards
Heinz!
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Old 11th February 2010, 05:26 AM   #7
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Okay, finally came up with the service manual and schematic for this boat anchor.

Would you believe +/- 300 volt rails for the output stage? And fully regulated +/- 240 supply for input/driver stages. Ouch. And by far the most parts-rich output stage I've ever seen; literally dozens of driver-type bipolars in arrays of series-parallel like I'd never imagined. Probably typical for early-mid 1970's technology given the limited voltage and current ratings available at the time; a modern MOSFET stage would surely be far simpler.

Quasi-complementary output stage in extreme totem pole; mostly 2N5657 with stacked 2N5657 drivers. Everything's four deep. Also some 2N5401/2N5551 devices.

I think the first time I apply power to this thing, it'll be at the end of a long extension cord. Outdoors.
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Old 11th February 2010, 07:21 AM   #8
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Old 11th February 2010, 07:41 AM   #9
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I'll need the requisite heavy rubber gauntlets and boots too, like linemen wear when servicing power lines.
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Old 11th February 2010, 12:27 PM   #10
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Quote:
Finally came up with the service manual and schematic for this boat anchor.
Any chance we could have a look too? Would be great, thanks.

Samuel
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