Crown d150 maintenance and upgrade

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Hi y'all,

I just got a Crown D150 to power my studio monitors (Tannoy dual concentric with the brown crossover).

The amp is clean, working and 100% original, never had any maintenance.

I plan on bringing it up to specs to prevent any faults, I'm gonna start by reccapping it.

I'm ok with studio electronics but don't have much experience with power amps.

I'm looking for advice from people with experience using and fixing these, What other parts should i replace to bring it back to specs? Any common faults i should be looking to prevent?

I know this amp is dc coupled, making me a bit nervous for my speakers. Should i add back-to -back capacitors on the speaker line to block dc as a protection? Or what other solution is recommended for protection against dc? (the Tannoy's crossover has a cap in series for the tweeter but not the woofer).

I also eventualy plan on upgrading the ua739c ic to something like a ne5532. i can easily make a 14 pin to 8 pin adapter but i'm not sure about what to do to adapt the circuit yet.

Any info would be very helpfull!

Ua739 were not a typical op-amp and you can not drop in a newer chip. See:
ua739 Op Amp Replacement
If memory serves, years ago I wanted to replace the ua739 in a DC300 and had to give up. I should think that someone has managed this feat but I have not found them. It would require redesigning the circuit, and probably using an externally compensated op-amp, certainly NOT an ne5532.
But I would probably replace the whole amp circuit (board) and use the power supply and chassis. The crown D series amps date from a time when silicon power and ICs were new and a lot of compromises were used to make them work.


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The '739 Crown D amps were brilliantly and holistically designed around the parts they could get. The '739 is NOT a limiting factor. The main gain of a D-amp is further along. Adding a hot-rod opamp to that would just be unstable. The '739 is a simple thing with few faults (not the many faults of more complex chips).

Just about every amp today is DC-coupled.

The Crown D-amps stood heads and shoulders above the pack in their day and for some years after because they were stable, clean, and RELIABLE. When transistor amps had a bad reputation, the Crown Ds gave no trouble. They sure had little tendency to throw DC on a speaker. I have run them into large costly midrange drivers fearlessly no-cap.

I have not had my nose in a D in decades. I recall the parts (in early models) were exceptionally high quality. I could believe those main filter caps are still in excellent shape. A big dummy-load and test rig could check that. These amps should exceed rated power all day and night. If power is down or THD is up, yes it could be decayed main caps. These amps are very tightly packed inside and unless you are deeply experienced I would not be randomly re-capping. The danger of messing-up exceeds the likely benefits.

The sound of the early Crowns is a matter of taste (and maybe age). I always trusted what I heard in a Crown. However more "modern", potentially "smoother" or "crisper" amplifiers are readily available today at prices far less than we paid (in real money) for those Crowns.
Allright, thanks guys.

I read pretty much everything on here and the net about these and nobody seems to have succesfully upgraded the 739 so i will leave it.

I got this amp because it was a stapple in a lot of recording studios here in Canada.

The d150 only has 3 electrolytics per channel, including one bipolar for input coupling. (appart from two large psu caps). i will check if they are easily accessible, i am usualy good at recapping but wont take risk of messing things up. i will probably leave the big psu caps in for now.

About possible fault throwing dc at the speaker, I could put caps in series to prevent it, I'm not sure how much it would degrade sound quality (phase shift?), but i'd rather be safe than sorry, after all, this particular amp is 50 years old.
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Back to back 10000 uf capacitors sound funny at high frequencies and low wattage. I run my music room system at 1/4 w base level, so I can hear distortion on top octave grand piano.
It is safer to put DC detect circuit on speaker output and a nfet disconnect. Two nfets source to source, drains to drive & speaker, isolated drive gate to source. Isolated drive typically a photovoltaic fet driver like a Panasonic APV1122 (from digikey). Current rating of fet greater than current to speaker, voltage rating higher than rail voltage.
DC detector a 12k resistor from OT emitter line driving a 2.2 uf ceramic capacitor, that connection driving a 8 v trip like line to line 7.5 zeners or a powerex bs08d diac. output of the trip line driving the input of the APV1122. Speaker ground the other side.
I don't know why ebay/amazon speaker protector boards always have a *****y metal contact relay that will weld the contacts closed in a serious fault. Other than it is cheap. Nfets are cheap, salvageable from switcher power supplies, but APV1122 is about $8.
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