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Old 23rd December 2013, 06:06 AM   #1
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Default ward beck m621. 20w broadcast amp opinions

Hi y'all! I was in need of some little amplifiers for monitoring in my home studio. And found those for cheap.

They are ward beck's m621. Ward beck is a canadian broadcast equipment company specializing mostly in broadcast recording and mixing consoles in the 1970-1980's. Highly regarded in the recording world.

I found those for very cheap out of the cbs/radio Canada studios. They are mono block, 20w linear amplifiers. Eack with a dedicated power supply and balanced inputs.

The specs seems good but i am not very knowledgeable in amplifiers topology ect, i would like the opinions of some of you guys.

Here's the service manual and schematic:


I'd like your opinions on the specs and the circuit topology if possible. And to know if it's a worthy amp compared to some of the best solid state ones.

Thanks alot folks
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Old 23rd December 2013, 06:24 AM   #2
Mooly is offline Mooly  United Kingdom
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OK (my opinion)... so "compared to some of the best available"... hmm... how shall we say... hopelessly outclassed. An LM3886 chip amp would easily outclass this one. The specs are actually really poor compared to modern offerings. The use of the 709 opamp drags it right down... OK, well that can be substituted.

It has curiosity value, it could be improved, but is it worth it... probably not.
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Old 23rd December 2013, 07:29 AM   #3
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Thanks Mooly!

I'm actualy very intetested in any mod/upgrade that could be done as i don't have budget for any hi-end amp or building one for now. Also i am not an audiophile and these would just be for second monitoring system in my studio.
I was mostly drawn to for the build quality and the balanced input. (Very practical in my noisey studio environement, and easier to interface).

I was already planning on recapping it, maybe i could add more capacitance somewhere if it could improve it.

@Mooly do you have any ideas on what i could do to improve it? Also what kind of opamp would make a better substitute to the lm709?

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Old 23rd December 2013, 09:16 AM   #4
Mooly is offline Mooly  United Kingdom
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The 709 and 739 are ancient with the 739 using an outdated package (14 pin) so you have problems there for a start.

That said it should be possible to fit somethiung like an OPA134 for the 709 and either two 134's or a single OPA2134 on an adapter pcb to replace the 739. You would have to remove the input and output compensation networks as the modern opamps don't use them.

I looked at the spec of the 709. 0.25 volts/uS slew rate and a pure class B output stage
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Old 23rd December 2013, 10:41 AM   #5
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Ok..eew . thanks Mooly idid some researchon how to substitute these parts.

I found an adapter to replace the metal cans to-99 lm709 toa dip-8. Shiyldnt be too hard. Im only worried cause they may have usrd these package for thermal disipation. Maybe i can find some opa134 in to-99 or a similar one.

Adapting the 14 pins ua739 willlijely be more of a challenge,but i googled a bit an it seems a bunch of people restoring Crown an Marrantz amplifiers have done it. So its possible.

I will look into it, thanks!

Anymore tips on improving this amp are welcome.
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Old 23rd December 2013, 01:53 PM   #6
Mooly is offline Mooly  United Kingdom
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No problems with dissipation. I just question whether its worth the effort tbh. The design has a lot of "SOT" (select on test) parts which will need a scope and sig gen to optimise after fitting new opamps and so on.
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