25' unbalanced connection between amp and pre-outs

[DVDdoug]

The amp does have balanced and unbalanced inputs.

Good! It's the inputs that are most important. Simple adapters will work on balanced inputs. It the balanced-differential input that ignores common-mode and ground noise. You don't need any kind of fancy "converter". (Creating balanced outputs is more tricky.)

I'd try RCA-to-RCA first and if you have hum problems get some adapters for the amp inputs.

 

[Zapper]

The amp does have balanced and unbalanced inputs.

Then either of my reply in #4 (RCA-XLR adapater + XLR cable) or @sam_adams in #5 (STP cable) would be a low cost improvement over RCA cables.

 

[Zapper]

Good! It's the inputs that are most important. Simple adapters will work on balanced inputs. It the balanced-differential input that ignores common-mode and ground noise. You don't need any kind of fancy "converter". (Creating balanced outputs is more tricky.)

I'd try RCA-to-RCA first and if you have hum problems get some adapters for the amp inputs.

No, you want the converter at the RCA output, and a differential cable in between. Then any hum and noise pickup is developed on both high and low conductors equally, which is cancelled out at the differential input. If you run a single ended cable, then convert at the amp input, you achieve no rejection of hum and noise, it all ends up on the high input.

See the diagrams that @sam_adams showed in post #5. The STP cable does the SE to diff connection at the RCA side. Likewise you can put the RCA to XLR adaptor on the RCA output, then run a XLR cable to the amp, which gives the same circuit as the STP cable.

 

[Roland68]

(Creating balanced outputs is more tricky.)

How much easier should it be than with DRV134 or Those1646?

 

[Zapper]

How much easier should it be than with DRV134 or Those1646?

It's a DIY project, which is tricky for some, and time consuming for anyone.

 

[Speedskater]

Getting back to long RCA interconnects.
a] Coax cables with heavy braided shielded are best.
Like: Canare LV77S, Belden 1505F & 1695 and Blue Jeans Cable LC-1.
But 5 cables all from one unit to another unit reduce the need for heavy shielded.
The heavy shield is more about amp to speaker cables.

b] no need to worry about space to AC power lines.
Unless the power line is to a popcorn popper or teapot.

c] Cables with twisted signal conductors are best. But this doesn't apply to coax cables or multiple cables.

 

[Roland68]

It's a DIY project, which is tricky for some, and time consuming for anyone.

Or you can buy ready-made boards. There are various ready-made DRV134 boards (10-20 $/€) for 2 channels, e.g. from Audiophonics or Aliexpress.
Some even come with cables, so you can even work with terminals or crimp contacts without soldering. Depending on the model, a single or +/- voltage is required, also easy to get.

Alternatively, there are boards with XLR connections and power supply. Simply strip and connect two short RCA cables, connect the transformer/plug-in power supply, shrink tubing or housing around it. Can be done in half an hour, 60 minutes at the most.

Many things are as easy or difficult as you do it yourself.

Addendum:
I almost forgot about these two boards, so it's even easier.
SE to Balanced
Transmitter/receiver SE to Balanced/Balanced to SE

 

[Roland68]

Getting back to long RCA interconnects.
a] Coax cables with heavy braided shielded are best.
Like: Canare LV77S, Belden 1505F & 1695 and Blue Jeans Cable LC-1.
But 5 cables all from one unit to another unit reduce the need for heavy shielded.
The heavy shield is more about amp to speaker cables.

b] no need to worry about space to AC power lines.
Unless the power line is to a popcorn popper or teapot.

c] Cables with twisted signal conductors are best. But this doesn't apply to coax cables or multiple cables.

In such situations I have had good experiences with Star-Quad cables, e.g. Sommer Cable or MOGAMI NEGLEX W2534.
Combine the two opposite conductors together. Two for signal, two for ground, and place the shielding on one side (source) to ground.

 

[Speedskater]

In such situations I have had good experiences with Star-Quad cables, e.g. Sommer Cable or MOGAMI NEGLEX W2534.
Combine the two opposite conductors together. Two for signal, two for ground, and place the shielding on one side (source) to ground.

But a simple coax with a heavy braided shield will be better. The lower the end-to-end resistance of the cable shield/ground the better.

 

[Doodski]

But a simple coax with a heavy braided shield will be better. The lower the end-to-end resistance of the cable shield/ground the better.

I agree if using a simple cross sectional cable like a coax with low resistance braided shield and a thicker solid core. I used 3, 3.5mm headphone extension cables to extend the lead of my earbuds from the notebook to my bed to a total length of about 18 feet + the earbud's cable length to ~21 feet. It worked although upon comparison the extended wire length combined with the cheap construction made the sound fuzzy, very slightly distorted and not very good for critical listening. So in the case of the OPs needs one should be very careful and use good quality stuff to avoid degradation of the signal integrity and not get fuzzy distorted sound etc.

 

[KSTR]

The DRV134 doubles the signal level (diff out gain = +/-1). That gives you a +6dB increase in signal level, so potentially a +6dB SNR improvement.

As would increasing the signal level on an unbalanced line. So no intrinsic benefit.

 

[KSTR]

I assumed that there was a real symmetrical input at the power amplifier, otherwise the whole thing would be meaningless.

Yes, sadly, there are balanced inputs that don't do proper subtraction (re-referencing to the local ground), usually in boutique "all-balanced throughout" circuit designs.

 

[pma]

Simple adapters will work on balanced inputs.

This is oversimplification. They work only in case that the source output impedance has been matched by inserting a resistor into -signal wire.

 

[KSTR]

This is oversimplification. They work only in case that the source output impedance has been matched by inserting a resistor into -signal wire.

View attachment 357818

While technically true that any balanced source should have balanced output resistance, I've found it has little effect in practice.
Removing the ground connection from the signalling does like 95% and the impedance balancing does the remaining 5%. In very harsh environments and for very long distances and less than perfect cable shields it may make some real difference, though.

 

[pma]

My experience says that in the current world of 50-100ohm output impedances and 2kohm input impedances the impedance matching does a lot. Yes it was not such a case when input impedances were 47kohm. I hope I do not need to post plots, again .

 

[Blumlein 88]

My experience says that in the current world of 50-100ohm output impedances and 2kohm input impedances the impedance matching does a lot. Yes it was not such a case when input impedances were 47kohm. I hope I do not need to post plots, again .

I would think 2 k-ohm input impedance is still a rarity in gear used with RCA sources. Like in this case an AVR out to feed a MCH amp. Are there that many amps out there with below 10 k-ohm input impedance? I like to see things done properly and that resistor is the right way, but I know of plain adapters working just fine in many cases.

 

[pma]

I would think 2 k-ohm input impedance is still a rarity in gear used with RCA sources. Like in this case an AVR out to feed a MCH amp. Are there that many amps out there with below 10 k-ohm input impedance? I like to see things done properly and that resistor is the right way, but I know of plain adapters working just fine in many cases.

But my post was about the RCA - XLR adapter cable …….
And low XLR input impedance is now very popular, see the no-input-buffer mania.