Balanced Output Phono Stage

[Reid Malenfant]

While i can't exactly get a move on with other projects this one was coming on pretty well until recently :) This one is pretty simple & doesn't involve me sorting out the mess in the garage so i can use my table saw :lol:

What we have here is a subtly modified Signal Transfer Company phono pre amp (http://www.signaltransfer.freeuk.com/RIAAbal.htm) that was designed by Douglas Self in 1996. Originally it was part of his precision pre amp but it has been released as a stand alone unit & frankly i'm incredibly greatful for this.

The thing is i'll asap be fitting an Audio Research MP1 pre amp into my system & there is no provision for any phono input, only line level ;) Because my equipment rack is a bit of a PITA to load stuff into i needed a quick fix solution in the way of a balanced output phono pre amp so i don't need to strip everything down & rewire when i come up with my own solution to this dilemma.

This will do as a stop gap, though i'll be developing my own phono stage in the future & having thought about it i might as well prepare for that now. Thanks to Andy (Teckno Commander) & his link to a site that sells some rather nice cases i'll be investing in a case that'll house everything very shortly.

This pre amp will be powered by a rather large bunch of NiMH (Nickel Metal Hydrite) batteries so as to avoid any mains hum on the supply rails. I have found a nice load of 32 AA 3000mAhr batteries for the princely sum of less than £12 new :) I'll use 30 of them & i'll have just under the voltage rating of the op amps used in this circuit. With regards to current draw i should be able to listen to vinyl for 40 hours before the batteries need recharging & they'll be recharged each time i turn the unit off. I'll need to make my own charger but this isn't a problem!

All the electrolytic caps in the signal path have been swapped for Sanyo Os-con caps, the 20Hz 3rd order 18Db/octave filter has had resistor values recalculated so the low pass is now at 15Hz & the moving coil input stage has been modified (or will be shortly) to reduce noise by a further 3Db. PSU caps have been ugraded to Panasonic FC capacitors & value increased from 220uf to 2200uf.

Unfortunately i must have had a bit of a blonde moment when i ordered the larger value Os-cons as i bought only 8 & that leaves me 4 short in the signal chain :doh: Hopefully the others i purchased as soon as i realised my error will be here in a week or so like the first lot.

So here we are at the moment...Bought as a kit & has been populated by me with as i say, subtle improvements :eyebrows:

http://i939.photobucket.com/albums/ad240/speakermark/Phono%20pre%20amp/phono1-1.jpg

http://i939.photobucket.com/albums/ad240/speakermark/Phono%20pre%20amp/phono2-1.jpg

Here is the case that there was absolutely no mention of in the ebay listing :scratch:

http://i939.photobucket.com/albums/ad240/speakermark/Phono%20pre%20amp/box1-1.jpg

http://i939.photobucket.com/albums/ad240/speakermark/Phono%20pre%20amp/box2-1.jpg

Needless to say i'll no longer be using this as there is a lot better out there & i'll be buying it soon :)

The capacitors in the high pass filter have also been change to 400V versions as apparently this will reduce the distortion produced by the HPF & all have been put on an LCR bridge & i now have 6 within 1% tolerance, all good news.

Here is part of the circuit (http://sound.westhost.com/project25.htm) (the first circuit diagram), i was worried about copyright infringement but someone has already posted the MC part of the circuit. To reduce noise further i'll be using 6 x 2SB737 in parallel & have already altered various resistors & a capacitor to suit the new situation. The addition of the extra 3 transistors should reduce noise by a further 3Db, might not seem a lot but it all helps ;)

[Reid Malenfant]

Back to it :eyebrows:

Today i ordered a nice new case for this project as well as a few other bits & pieces. I decided to put everything in a single case as it'll make life a lot easier, the batteries i have (32 x AA NiMh 3000mAhr) won't fit in the case that i was supplied with to put the phono stage in & i'll be damned if i'm going to purchase a better looking case for the PSU :rolleyes:

The new case that'll be on the way shortly is this one (http://www.modu.it/pesante.html) with a 10mm thick silver front panel & no handles :) Cheers again to whoever it was on here that pointed this firm out to me.

While i'm about it i decided that i'd like a balanced input from the phono cartridge. This is pretty easily done without resorting to using a differential amplifier (which would increase noise) by using a pair of transformers. The type i selected were an old pair of Partridge 977 which have a step up ratio of 6:1. If i remember correctly Jerry is using the very same type. It'll just be a case of altering the way they are wired up & i'll have a balenced input & the transformers will convert this to single ended & feed the MC input stage.

This is very handy indeed as the resultant output (with X5 gain) will be approximately 9mV before it hits the RIAA amp. The original output of the whole unit was a little on the low side for my liking & now it'll be doubled which suits me fine ;)


I really ought to point out that the only reason i bought this kit was so i could get my Audio Research MP1 pre amp installed. This unit has a balanced output & obviously the MP1 has a load of balanced inputs. So as soon as this phono pre amp is up & running I can get my rack sorted with the new kit. At a later stage i'll design & build my own phono pre amp & remove this one whilst leaving the PSU & everything else the same :) It's just to get the ball rolling in other words...

The 3000mAhr batteries will be recharged each time the unit is shut off even though they won't need to be. NiMh cells have no memory effect so it'll be safe to do that. The batteries would probably last approximately 60 hours before they'd be fully discharged with this pre amp. The one i'll design will be totally discrete full class A & will no doubt eat some current, however i see no problem in them lasting at least 12hrs+ with whatever circuit i come up with.

Back with more asap :wave:

[John]

I would like to hear this when finished as I might go down a similar route

[Reid Malenfant]

You aren't the only one John ;) Trust me i have had the MP1 sitting here for over a year & the only reason i haven't installed it is that it's a real pita to wire all the gubbins up in my rack :rolleyes: There is no phono pre amp in the MP1, it's all line level, so i decided to wait until i have a seperate phono stage completed. Balanced output versions tend to be kind of expensive, let alone balanced input phono pre amps :lol:

Once this is done i'll be laughing :)

Once completed & the MP1 installed you are more than welcome to come & have a listen :) you can even bring your own vinyl as my collection is not that large (though this might change once this is up & running!).

[John]

Hi Mark
So what tt will you be using when up and running
It should sing well

[Reid Malenfant]

I have got an Aiwa AP2600 which is very similar though possibly a tad better built than a standard Techie 1200/10 & a hell of a lot rarer, i can't even find a picture of one other than my own (http://theartofsound.net/forum/showpost.php?p=139262&postcount=1) :eyebrows: Cartridge will soon be a Denon DL-103 courtesy of hugo (shuggie), that should arrive tomorrow before 1PM :)

All i can really tell you is it weighs in at nearly 3 stone, it's a quartz lock direct drive deck with variable speed a la Techie. It actually tells you the speed in +/-% as you alter it (not that i do). Oh, it sounds quite nice as well ;) & just to round it off i bought it from a car boot sale for £20 :lol:

[Reid Malenfant]

Well the case arrived today & quite nice it is to :) Fairly quick delivery from Italy & certainly well packaged, i'll definately be using this firm in the future if i need any cases that's for sure ;)

One slight problem has made itself apparent though :eyebrows: Like a plonker i didn't think about mounting a mains switch on that nice fat 10mm thick front panel :doh: Problem solved though as i have found a nice quality DPDT illuminated pushbutton switch that will mount on a panel up to 18mm thick.

I just have to wait for it to get here.........

From Hong Kong :doh:

:lol:

Well at least it gives me time to get other stuff done & there is plenty to do ;)

Back on this one when things begin coming together :)