A75 mono

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I would like to have your oppinion on building the Passlabs A75 as two monoblock. I'm thinking of building it in the same chassis but divided in two separated region.
Or do you think I will obtain a better result if I build to separated monoblock.

Monoblock or dual-mono design are used to reduce crosstalk between stereo channels. My opinion is if there is perceptible crosstalk in a stereo amplifier sharing the same power supply, it is likely resulting from other (majors) errors in the amplifier design. A good design like the A75 will not suffer from this problem. Of course, wires in your chassis, must be carefully routed.
some worry

With Class A/B designs which pull variably off the rails in response to the signal, there is some worry that without a seperate supply a high draw from one channel will cause the other channel to run less optimaly. There is also crosstalk with some signal appearing on the other channel.

However, Class A amps like the A75 pull steady current off the supply so a supply designed properly to take current from both channels should have none of the first problem described. As to crosstalk, yes, monoblocks will give you better performance, but how much?, that may not even been measurable by you.

I read that one man disliked monoblocks as he felt the lack of mechanical connection between the two channels made them slighty different and they sounded "wrong" to him..

Monoblocks are a great way of dealing with the heat, and in my opinion they look darn cool next to each speaker with some big cable.

I'm sorry but your statement regarding Class-A current draw is incorrect (Quote: 'However, Class A amps like the A75 pull a steady current off the supply....).

As I explained in another thread (Zen v Trimodal), the only amplifier topology that presents a constant load to the power supply is a single ended Class-A with a constant current source and only a single supply rail. In all other cases, either one or both of the supply rails will have a modulating current when the amp is under load.

The A75 is a push-pull Class-A operating from split (+/-) supply rails. The current in each rail, under load, will vary from near zero to twice the quiescent current. This is the same variation (in actual current value) as that for a Class-B amplifier giving the same output power. The difference between the two is that the Class-A has a full sine wave current waveform and the Class-B a pulsed half sine wave waveform (assuming a sinewave input to the amplifier).


[Edited by Geoff on 04-15-2001 at 05:01 AM]
Stereo vs. monoblocks

One solution to the stereo vs. monoblock problem which I have used,is to use one common transformer, but separate rectifiers and capacitor banks. This somewhere inbetween as a tech. solution, but the bulk of the impulse load is anyway supplies by the capacitors, and crosstalk problems are virtually eliminated. Even further, the transformer can have 4 secondaries, or 2 center tapped, although sharing the secondaries between the channels is essentially OK.If one builds for "dual mono" it will also save some weight and a few bucks.....
dual vs 2 mono blocks


I started building a dual mono Class A amp (the JLH design) into one case. In the process I've encountered two (minor) points:

the PCB layout is the same for each channel while the placing of some components, like output devices, might restricted to 'left' or 'right' relative to the PCB.
I'll just keep all the wiring to those the same lenght for both channels.

is there enough room on the case to fit all the heatsinks?
I think I have solved it. But I only can tell after completion.
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