Thinking about Going with 2 Monoblocks and a Preamp

[musicforcities]

I just recapped my Marantz ma-500u monoblocks. I love the form factor, you can fit directionally in a 19” rack. they take up so little room for a 100-120w class AB amp and the sound is great—flat/neutral and zero hiss from my 84db speakers at max volume. The only thing they lack is a 12v trigger. The ma61000 are basically the same but with a trigger.

I did replace the power cords— and used the end of 3’ 2/ 14AWG cord instead of the 6ft long 16awg it comes with. not for snake oil reasons, I just wanted shorter cords and I had these extension cords laying around.

The bias voltage is rock steady and zero dc offset at binding posts. I mean zero. They don’t get hot in use either. Just warm.

They are elegantly laid out inside with plenty of room between pads for soldering. Not to many el caps to deal with and the pcb are of good quality. The only trick is the leads were all bent flat to the board at the factory. So you can lift a trace pad easily. I pumped the solder out and then clipped the caps leads on the other side of the board and that worked well.

The original caps are all Elna and most audio Elna. Nevertheless, they are all 85 degree caps and are a few are known to be prone to failure or to drift out of spec in the hotter spots. Including the filter caps. In one amp both Elna filter caps were bulging and had higher esr and lower capacity than the other. So I changed those. Of course one can’t find the exact caps anymore and the ones that do exist that fit are all 35mm rather than the original 30mm. So you have to drill new holes to accommodate. But the board is good, the filter pads are thankfully huge and a sharp drill bit did the job without any issue in minutes.

I refreshed all the thermal paste and pads while I was there as you have to detach the heat sink from the transistors to get to most things. A good preventative maintenance thing to do on any amp over 10 years old as thermal paste becomes less effective over time and sil pads can’t be reused anyway.

 

[RayDunzl]

What are you trying to buy with the $350 budget?

Wire or gear?

 

[musicforcities]

Right now, turn on your setup, walk to your chair.

If you have monoblocks, and they are 30 feet away from the preamp and you have the monoblocks next to the speakers, its

Turn on your preamp, walk to one monoblock, then the other and then your seat.

What you need is for the preamp to power on your monoblocks, and 12V triggers are largely what ends up being used. Surprisingly hard to find minijack to minijack cables that are 30 feet!

Maybe because there would be too much power lost on 30ft trigger cables (just thin mono trs headphone cable usually. But You can use any 2 conductor 20-18awg and two of the trs jacks with screw terminals (available everywhere.

However…is it not generally preferable to have shorter line level runs and longer speaker runs anyway?

Because line level is low level and can pick up noise more easily at 30’ than relatively high current 12awg speaker wire. And there is no issue of signal level loss assuming you have appropriately thick speaker wire.

Also less chance of ground loop or ground noise if the preamp and amps go to the same socket.

In other words, the amps usually should be near you preamp.

 

[Speedskater]

However…is it not generally preferable to have shorter line level runs and longer speaker runs anyway?

It depends on:
a] how short is short?
b] how long is long?
c] is the interconnect system: Unbalanced RCA or balanced XLR?
d] what is the loudspeaker's impedance with frequency curve?
e] are the amps and pre-amp on the same AC power circuit?
* * * * * * * * * * * * *
50 foot RCA interconnects can be very long, especially if the amps and pre-amp are on different AC circuits.
XLR interconnects can be hundreds of feet long.
if the loudspeakers have a real nasty impedance curve, then 10 foot long speaker cables might have a frequency response effect.

 

[GXAlan]

However…is it not generally preferable to have shorter line level runs and longer speaker runs anyway?

My understanding is that it’s the opposite of what you think. You would imagine that low voltage line level is at risk and higher voltage speaker runs could handle longer distances… power transmission lines are high voltage after all.

On the other hand, if you look at Kenwood, they used to advocate for running 12 meter runs from the preamp to the amp and keeping amp to speaker distance less than 1 meter with their monoblocks and Mark Levinson and Bryston mention the same thing.

Benchmark states that the speaker cables are really what limits the damping factor. 10 ft cables are one thing, but if you are talking 12 meters (36 ft) then it probably makes a difference.

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

 

[antcollinet]

My understanding is that it’s the opposite of what you think. You would imagine that low voltage line level is at risk and higher voltage speaker runs could handle longer distances… power transmission lines are high voltage after all.

On the other hand, if you look at Kenwood, they used to advocate for running 12 meter runs from the preamp to the amp and keeping amp to speaker distance less than 1 meter with their monoblocks and Mark Levinson and Bryston mention the same thing.

View attachment 334394

Benchmark states that the speaker cables are really what limits the damping factor. 10 ft cables are one thing, but if you are talking 12 meters (36 ft) then it probably makes a difference.

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

I would say that is bad advice.

Low level interconnect (especially unbalanced) is much more suscptible to cable length than speaker interconnect. The only thing you really have to worry about with speaker wire is keeping the resistance low - so use larger diameter conductors for longer runs.

 

[egellings]

Why the monoblocks? Little to be gained that way. Unless you just like additional boxes and space taken up....

With mono-blocks, you get zero crosstalk due to the complete physical isolation of the signal circuitry and separate power supplies. Thing is, though, does music listening require that level of isolation? Also, if the preamp causes some mixing of the channels, then separate power amps will not help that. The big benefit of using mono block amps is that the power amps can then be sited right next to the speakers, which allows the use of very short power-handling speaker wires. It's easier to send a small signal via coax cable to an amplifier than it is to send a lot of power via speaker wire to speakers.

 

[antcollinet]

It's easier to send a small signal via coax cable to an amplifier

Not really - typically more expensive** and more susceptible to noise/interference than speaker interconnects. You also have to run long power cables to the power amps located close to the speakers.

(**in the real world, not the audiophile world where speaker cable costs megabucks per meter)

 

[egellings]

Audiophile anything will be expensive for sure. But standard RG-58U works well, and I have two 20-foot lengths in my setup, preamp outputs to power amps sited near the speakers. My home-brewed preamp seems to have no trouble driving the cable. I verified that with frequency response & distortion testing with and without the cable connected to the preamp. It is true, however, that I live in an electrically and RF low noise area, so I am not taxed with that problem. If that were the case, then I'd go with balanced small signal cable. Speaker wires can act as antennas for RF garbage and could conduct that noise into the system. I lived in a suburban area adjacent to Milwaukee WI USA (Shorewood), and there is an antenna farm there on east Capitol Drive that broadcasts commercial TV and radio signals. I could pick up WTMJ with the filling in my wisdom tooth! My stereo setup was simply unusable, since I could not keep the RF out of it no matter how much shielding or how many ferrite beads I used. I ended up moving.

 

[antcollinet]

Audiophile anything will be expensive for sure. But standard RG-58U works well, and I have two 20-foot lengths in my setup, preamp outputs to power amps sited near the speakers. My home-brewed preamp seems to have no trouble driving the cable. I verified that with frequency response & distortion testing with and without the cable connected to the preamp. It is true, however, that I live in an electrically and RF low noise area, so I am not taxed with that problem. If that were the case, then I'd go with balanced small signal cable. Speaker wires can act as antennas for RF garbage and could conduct that noise into the system. I lived in a suburban area adjacent to Milwaukee WI USA (Shorewood), and there is an antenna farm there on east Capitol Drive that broadcasts commercial TV and radio signals. I could pick up WTMJ with the filling in my wisdom tooth! My stereo setup was simply unusable, since I could not keep the RF out of it no matter how much shielding or how many ferrite beads I used. I ended up moving.

I think if you are using balanced interconnect, you'll have few problems with longer cables to the amps. But you still have the long power cables to contend with - and balanced interconnect, together with that power cable cost is always going to be pricier than basic functional speaker cable of the same length.

 

[egellings]

I think if you are using balanced interconnect, you'll have few problems with longer cables to the amps. But you still have the long power cables to contend with - and balanced interconnect, together with that power cable cost is always going to be pricier than basic functional speaker cable of the same length.

True.