Mod to just use power amp section of my integrated amp


Novice Member
Hi all,

I’m looking for some advice from electronics guys or gals please.

I have a MusicalFidelity M3si integrated amplifier. It works great and I’m very happy with it, driving a pair of Focal Aria 905 speakers.

However, having bought an RME ADI-2 DAC FS which serves as an excellent pre-amp with a powerful DSP, I thought I would try driving the power amp in the M3si directly. So I disconnected my AVR that was connected to the HT input and plugged the line-out from the DAC into it, then did some back to back tests. The outcome is that directly driving the power amp is markedly better - for example vocals are much more clearer - even a normally cynical family member spotted the difference without being told what to look for.

So - that’s the next mod for me to make. But the problem is I still want to make use of the HT input for my AVR. So I took the lid off the amp to see how it was wired, hoping it would be a simple relay switching arrangement that bypasses the pre-amp and connects directly to the power amp. If so, then I’m competent enough to replicate it and convert one of the other inputs to do the same. Unfortunately the input selection is via a microprocessor controlled analogue switch IC, so not possible without circuit diagrams and ability to change the code in the cpu to replicate how the HT input is routed. But it is also apparent that the HT input still goes through some aspects of the pre-amp, even if it is just the switch IC, which isn’t ideal for what I’m trying to achieve in bypassing all pre-amp effects.

Given this, the question I have is whether I can disconnect the leads that go from a socket in the pre-amp over to the power amp (a very obvious pair of 2 channel leads about 8 inches long) and build my own 2 in 1 out switch, using some quality signal latching relays that connect straight to it? Putting a relay in the signal path isn’t ideal but it may well be superior to use of a switching IC (separation / isolation for example).

I’d be grateful for any input from people who know.

PS I could buy the M6s PRX that achieves exactly what I want, but it’s overpowered for my needs and costs £2600!
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Distinguished Member
PS I could buy the M6s PRX that achieves exactly what I want, but it’s overpowered for my needs and costs £2600!
can go back catalog...the musical fidelity a5cr if cant afford the m6.. has a dual selectable inputs... there is nothing such as over powered. its about 50wpc more.. its more the power reserves ...

alternatively buy a musical fidelity integrated further up chain... a5 or m6i with better pre stages ... than m3i


Well-known Member
Could you not just use a manual 2-in x 1-out input selector switch to switch between the two sources (AVR/DAC) connected to the amp input?

OR connect the AVR output to the DAC input and connect the DAC to the amp where the AVR is currently connected. then you get to use both on the same input


Distinguished Member
Could you not just use a manual 2-in x 1-out input selector switch to switch between the two sources (AVR/DAC) connected to the amp input?

This was going to be my suggestion too.

I did a similar thoing with a NAD amp that had the pre and power sections on 2 pairs of adjacent RCA sockets normally joined with some solid links. In my case I use a 2 input RCA switch with the inputs connected to the preout on my AVR and the preout on the NAD itself, essentially the RCA switch was an external HT bypass.


Novice Member
Thanks for your replies.

I’ll take a look at the back catalogue - good idea.

I have ordered the parts for a switch £30 all in. I’d rather build one than get an obscure import (eg via Amazon), then I know the quality of its components. Also some of the switches have volume controls (eg the Schiit SYS), which I don’t want. There is the Decware ZSB switch made in the US but it’s $500 delivered.

schiit sys

The Decware Zen Switchbox model ZSB

Interesting that the NAD amp has the pre and power stages connected by an external jumper - suggests that is plausible just to connect the output of my switch to the wire joining pre and power (having disconnected it from the pre). I notice where this wire joins the power amp circuitry the first component seems to be a capacitor- perhaps to remove any DC.

Any other thoughts are welcome.


PS. On trying to decipher the circuit in the M3si it’s interesting how inexpensive the components are having looked them up on the web. Eg microprocessor, switch IC, mute relay all just a couple of quid each. And the pre to power wire is just a bit of thin wire - nothing special. I know it’s how it’s all put together that matters, but just shows not to fret too much over introducing something like a quality signal relay into the signal path.

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