Monogram 3100 class A+ amp

[turbodeas]

Here are a couple of pictures of the insides, for those that are interested.....

 

[turbodeas]

An update - just hooked it up to my Tannoy System 800's (via a cheap pair to make sure it doesnt blow anything up!) and it sounds pretty good to my ears. Sounds very effortless and powerful, with a surprising transparency for something fairly retro. It does get quite warm, but I suppose that's to be expected when (at least some) class A topology is involved?

I might look at giving it a service.

I'd still be interested to hear if anyone has any information regarding the brand or this model - it really seems quite rare.

Cheers all,

Chris.

 

[jerluwoo]

Some selected specs:

Power Output: 3100 : 105W RMS

Damping factor: >400 to 1KHz - 8 ohms

Input impedance: 22K at min. level 15K at max. level.

Sensitivity: 0.775 RMS 0dB/1.5 volt max. Internally adjustable

Stability: Unconditionally stable with any load or source impedance

Electronic Design: Pure Class A at 1 amp, class AB at 1amp - 5amps.

This tidbit of info would seem to indicate the driver transistors are run class a and the outputs are set up as class c in that they dont start conducting until the drivers reach a certain output current. Would explain why the OP found the driver transistors and heatsink to be hot while the outputs were cold with no measurable idle current. Those 2 5watt resistors in the center by the trim pots in turbodeas' pictures are the driver emitter resistors, needing to be large to deal with power/current to the speakers before the outputs kick in. Couldn't make out the values from the pics but most likely a much lower value than you would find in standard AB amps.

 

[turbodeas]

Hmmh, that's interesting stuff jerluwoo. Thank you for your input. I've continued to use the amp, and it sounds pretty good to me.

 

[shlauff]

Due to the fact that pretty much all the information on the internet about Monogram is in this old post, I'll post this lot here, rather than start a new thread.

I've recently acquired the mysterious Monogram 3000 & 3100 (pre and power respectively) and someone out there may be interested!

https://www.diyaudio.com/forums/attachment.php?attachmentid=750575&stc=1&d=1555604911

https://www.diyaudio.com/forums/attachment.php?attachmentid=750576&stc=1&d=1555604911

The Preamp is still working just fine (sounds great, though I suspect it could do with an overhaul given the age of these things) and opening it up it looks incredibly well put together. Lovely big ALPs stepper as the volume pot...

https://www.diyaudio.com/forums/attachment.php?attachmentid=750577&stc=1&d=1555604911

The Power amp I'm informed is in need of some kind of repair, I'll probably stick a meter on the output devices when I get the chance. The mystery of sorts is what on earth are the two switches tucked behind the left rack ear? I've noticed a label that says there has been a mod done, presumably the switches... It looks at first glance like they go back to the power supply, as the contacts are bare on the outside of the chassis hopefully these aren't live?!? ...I know health and safety wasn't quite as strict in the 70's, but come on!
https://www.diyaudio.com/forums/attachment.php?attachmentid=750578&stc=1&d=1555604911

https://www.diyaudio.com/forums/attachment.php?attachmentid=750579&stc=1&d=1555604911

Hopefully I can track down a schematic from somewhere and get it fixed up...

https://www.diyaudio.com/forums/attachment.php?attachmentid=750580&stc=1&d=1555604911

 

[shlauff]

Due to the fact that pretty much all the information on the internet about Monogram is in this old post, I'll post this lot here, rather than start a new thread.

I've recently acquired the mysterious Monogram 3000 & 3100 (pre and power respectively) and someone out there may be interested!

https://www.diyaudio.com/forums/attachment.php?attachmentid=750682&stc=1&d=1555669929

https://www.diyaudio.com/forums/attachment.php?attachmentid=750683&stc=1&d=1555669929

The Preamp is still working just fine (sounds great, though I suspect it could do with an overhaul given the age of these things) and opening it up it looks incredibly well put together. Lovely big ALPs stepper as the volume pot...

https://www.diyaudio.com/forums/attachment.php?attachmentid=750684&stc=1&d=1555669929

The Power amp I'm informed is in need of some kind of repair, I'll probably stick a meter on the output devices when I get the chance. The mystery of sorts is what on earth are the two switches tucked behind the left rack ear?

https://www.diyaudio.com/forums/attachment.php?attachmentid=750685&stc=1&d=1555669929

I've noticed a label that says there has been a mod done, presumably the switches... It looks at first glance like they go back to the power supply, as the contacts are bare on the outside of the chassis hopefully these aren't live?!? ...I know health and safety wasn't quite as strict in the 70's, but come on!

https://www.diyaudio.com/forums/attachment.php?attachmentid=750686&stc=1&d=1555669929

https://www.diyaudio.com/forums/attachment.php?attachmentid=750687&stc=1&d=1555669929

Hopefully I can track down a schematic from somewhere and get it fixed up...

 

[NATDBERG]

My scincere apologies for the wait .. (5 years...) but I have now scanned the paperwork I have! Currently trying to attach it and the photos to a post but lacking a security token.. or it could be I need to re-size to 72dpi!

 

[NATDBERG]

Attachments

 

[NATDBERG]

I've scanned them at 300dpi but had to re-size to load them up.. So I 've deposited the file here:

https://archive.org/download/monogramprepowerinfo/Monogram pre-power info.pdf

Right-click on the link and select "Save Link As.. " or the equivalent. Is a direct link to the file, not a viewing page.


Let's see how long it stays there.