Balanced circuits for Pre/phono/psu 2and3

[Henry Curniffe]

Good afternoon all.
Since joining this most enjoyable forum I have spent some time catching up on past threads on various component changes and circuit alteration of the venerable pre/phono/psu....2/3 units. The various routes taken so far seem to be in search for to achieve audible improvements/circuit efficiency etc.
As you can see from my antecedent declaration I am an electronics technical ignoramus however some of the discussions I have found to be useful.
Three is however one subject that I have read about in magazines and elsewhere that lends itself to the subject of audio improvement but not discussed here ,it is that of balanced circuit. I've read that the record player/cartridges lends itself to this naturally. I do not know the technical difficulties that this may present with these units but in the search for audio nirvana or valhalla there may be some merit to be achieved.
I wonder if anyone might have looked at this in the past and could share their findings.
Sorry for the ramblings and I look forward to your replies.
Henry.

[Greg.]

Hello Henry,

I am no expert on this subject but I have always rejected going down this road because those in the hobby and industry who govern my respect have always stated that for domestic Hifi, balanced systems are of no benefit. Apparently balance systems are certainly applicable to studios and other commercial operations, but not necessary for simple home use. Yes, I know commercial balanced products are out there and even some diyers have done it, but the wisdom seems to be, don’t waste your time or your money.

[Richard]

Hi Henry, all, you can certainly wire Pre3 outputs as balanced (mine were as there are 2 sets of outputs with both RCAs and mini XLRs supplied so I fitted both).

I've not used balanced though as I understood 1) that the benefits are in rejecting noise on long cables which I don't use and 2) that the other equipment in the chain would also need to be balanced.

Hopefully John Caswell will be along soon and shed some light from his professional days

[John Caswell]

Hmmm interesting thread.
I have just done something similar to a Phono3.
Converting the input to balanced is fairly easy IF you have a MC cartridge and balanced wiring a la SME 5 pin DIN to phono cable, plus of course a step up transformer. That is how I run my Phono 3 input. Trying this with a MM cartridge could be very difficult to impossible if the cartridge has the R/H signal return (green) connected to the body of the cartridge. In this case I would be inclined to use a proprietary balanced to unbalanced line receiver IC, of which there are many giving excellent results, but you would need a balanced ±18V DC supply.
The phono3 output can also be converted to balanced by using a 1:1 isolating transformer designed as a 10k:10K input isolating transformer. Surprisingly this works very well as it just just wired across the output of Phono3 after the output cathode DC isolating capacitor (2.2µF)
Converting Pre3 to balanced input is a different kettle of fish unless you only want one input eg Phono in which case a simple 1:1 10K:10K input matching/isolating transformer will suffice very well mounted in a small box with balanced input connectors (XLR or DIN) and unbalanced output connectors (RCA/phono). Once again instead of the transformer you could use the aforementioned IC converter. The main problem with transformers is cost and availability.
Food for thought so....any questions stick them up here and I and others will try to come up with solutions.

John

[bikerhifinut]

Heres a question.
I'm finally looking to put a version of pre3 together.
The idea of balanced output appeals to me as I like to put the power amps behind and close to the speakers and drive them from quite a long interconnect from the preamp.
So obviously I will have to "unbalance" the signal at the power amps, which probably means a transformer, and in view of the meaty nature of the pre3 output transformer I imagine something fairly substantial may be needed to cope with the potential drive from them? Or I redesign my power amps to be balanced all the way through by dint of replacing the phase splitter with a pair of driver valves etc as no need to split it so to speak.

Or..............

Will the simple act of isolating the preamp etc by using the transformer outputs give me a reduction/elimination of the low level (very low but annoying all the same) hum that so far I have been unable to solve when using a long cable to the amps. I dont know if its induced hum from the mains wiring etc or the old bugbear of a hum loop. I use good quality Microphone cable 2 core plus screen and the screen only earthed at the output sockets. preamp and power amps both have earth lift resistors above mains/chassis earth. The power amps are silent when a shorting plug is inserted into the socket.

Just wondered.

A

[John Caswell]

Now the fun begins!!
1) If you use the Pre3 as a balanced output, the screen of the twin screened cable should be earthed somewhere either at the send or receive end, helps reduce unwanted spurious noise ppickup
2) Ideally the input of the amplifier should be balanced either electronically or with a transformer (My personal choice)
3) Will it remove the hum? Haven't got the faintest, but using an input coupling transformer I would suggest yes. If you go fully balanced it will remove any earth/hum loop as the pre/power are only earthed via the power lines and not power/signal lines
4) No the input transformer is not required to be large as it is dealing only with audio not DC and audio as in Pre2/3. I would look at Sowter, Hammond, eBay, and any other quality manufacturer for a 1:1 - 10K:10K or 20K:20K transformer. Usually they are 10K, do not go any higher or lower. be prepared to pay for them! The output impedance of Pre3 is very low ca 10R and will drive pretty well anything including headphones.
Any further questions or comments ask here.

John

[John Caswell]

Now the fun begins!!
1) If you use the Pre3 as a balanced output, the screen of the twin screened cable should be earthed somewhere either at the send or receive end, helps reduce unwanted spurious noise pickup
2) Ideally the input of the amplifier should be balanced either electronically or with a transformer (My personal choice)
3) Will it remove the hum? Haven't got the faintest, but using an input coupling transformer I would suggest yes. If you go fully balanced it will remove any earth/hum loop as the pre/power are only earthed via the power lines and not power/signal lines
4) No the input transformer is not required to be large as it is dealing only with audio not DC and audio as in Pre2/3. I would look at Sowter, Hammond, eBay, and any other quality manufacturer for a 1:1 - 10K:10K or 20K:20K transformer. Usually they are 10K, do not go any higher or lower. be prepared to pay for them! The output impedance of Pre3 is very low ca 10R and will drive pretty well anything including headphones.
Any further questions or comments ask here.

John

[Richard]

Hi Andy, all,

There's a very readable page here,

http://www.bluejeanscable.com/articles/balanced.htm

The next to last paragraph of that page also deals with quasi balanced cables and explains they're a lose-lose (increase of capacitance and loss of shielding) so it may be worth trying high quality unbalanced coax instead.

The problem of hum though is that the low 50Hz energy is induced and gets through the thin Faraday cage type screen on unbalanced cables which blocks higher frequencies. Isolating the cables, or much heavier screening such as metal conduits, might help.