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Using 323s as MC head amp.

Chops54

Chops54

pfm Member
I'm thinking I can use the front end of a pair of 323s as an MC head amp for my valve phono stage. Looking at the circuit diagram I think I would need to remove R16 and take the output from Q3 there. I would cut the V+ track between Q3 and Q6 and take the supply straight to Q3. Would I need a decoupling cap on the output? If I'm talking daft just say so ;)
 
Hi Si
I did it with a pair of 523 boards a few years ago and they do sound very good

523 3 by Alan Towell, on Flickr

Cant remember exactly what I did but I remember removing the zenner, 2 x 2k2 resister , 68R resister 47uf decoupling cap Q4 , basically everything as a filter for the power supply as I used a 2 x TPR2 regulated supplies

Alan
 
Hi Alan, my favorite solid state phono stage uses a pair of 323 cards with the voltage regulator components removed and fed from a pair of ALWSRs. I prefer this stage to my NJ phono stage with either split NJ 323 cards or split Naim 323 cards. I don't know why that's the case but it just is. Anyway I decided to have a play with a few moving magnet carts so I set up another turntable and put a pair of split 322 cards in the NJ phono stage. A lack of gain was the first issue which PD helped me sort out but I'm afraid I just don't like it. After much messing about I built myself a valve phono stage based loosely on the 834P. This I really like so this is what I've been using for the last few months. Anyway I wanted to build an MC head amp so I could use my MC carts through the valve phono stage and thought why can't I use the front half of a pair of 323 cards? Q1, Q2 and Q3 seem to be the section of the circuit I need to use, the rest will be disconnected by removing R16 and breaking the +VE track just before Q3. I just need to know if taking the output straight from Q3 will work and would I need use a decoupling cap?
 
I'm not sure whose circuit diagram you are referring to.

You would have to figure out the gains, if the Naim MM and MC cards have similar gains for their second stages then you'd be 'on'. But the first stage of the MC card isn't intended to be driving anything general, so who knows what would happen.

If you're up for some deeper DIY then http://users.ece.gatech.edu/mleach/headamp/ and some soldering might be interesting.

Paul
 
Thanks Paul, here's the 323 circuit diagram. You can see the split in the circuit after Q3 at R 16 but I'm unsure whether to break the circuit before or after R16. I'm sure there would be enough gain to feed a mm stage from there.

Image from https://lh5.googleusercontent.com/-YFZX347TuuM/UgctgcCg8TI/AAAAAAAABLw/d3rQk09QJyE/s1280/323.jpg.

I've looked at the Leach circuit several times and it doesn't appeal. I'm unsure about cartridge loading. It does say later on in the text that this is possible but it also says about having RL at 1K so as not to load up the cart. The other MC head amp I've been looking at was put forward by PD.

Image from http://farm5.static.flickr.com/4146/5062835572_12693f14cc_o.png.

This never got of the ground but I believe someone may have made some boards for it?
 
Ah I see what you are trying to do now :rolleyes: sorry didnt read your first post properly:eek:
You just want a step up from the first stage ;)

Hopefully PD will be along to advise properly :D

I would leave Q3 in and fit a 10Uf output cap in the position of R16 and a 100k resistor on position C6
You would have to cut the power rails as well to separate all the other components in the circuit

Alan
 
Hi Alan, you've got it mate. I've just prepped a couple of boards to give it a try. I've removed R16 and broken the +VE track between Q3 and R18 and taken the +VE feed to the front end. That's a good call putting a 10uf cap where R16 should be and a 100K resistor in place of C6. I'll remove Q6 to disconnect the rest of the circuit or I'll just cut the track.
 
I think the gain of the MC front end might be too high, but I can't figure out the number at the moment. This only matters if you start to overload the MM input it's bodged on to.

I reckon knock PD's head amp up on perforated board and see what's what.

Paul
 
Chops54 said:
The Lentek has quite a following Alan. I believe Nick Gorham has done a bit of fettling on it too. I would have gone with the Hagerman Piccolo but he doesn't do half kits any more. It's a very flexible MC step up.

Here's a link to the original PFM thread if you're interested.

http://www.pinkfishmedia.net/forum/showthread.php?t=86812
Click to expand...

The Hagerman Picalo looks great as well and as you say flexible

I have a little lentek so Ill look up what Nick has been doing to them over on TAOS . Not sure if I can do anything to mine as I think its fully potted?.

Alan
 
Paul R said:
I think the gain of the MC front end might be too high, but I can't figure out the number at the moment. This only matters if you start to overload the MM input it's bodged on to.

I reckon knock PD's head amp up on perforated board and see what's what.

Paul
Click to expand...

Cheers Paul. Haven't got it to work yet. I took the output straight from the emitter of Q3 through a 2.2 uf film cap but there was virtually no output so I'm going to try it later as Alan suggested. Maybe I don't need a cap there?

I thought about doing PDs circuit on perf. I'm not that good at this kind of stuff but I'm seriously thinking about it. It might take me a while lol. I did do the Le Pacific head amp using a single 2sk170 per channel and that worked ok but I found it very strident sounding and not to my taste at all. I tried different loadings but I think that also changes the operating conditions for the fet.

Alan, re the Lentek, I think someone managed to de pot one. Not sure if it was Nick or not. Marco uses one doesn't he? There is a detailed thread over there.

I might email Jim Hagerman if nothing comes out of this. He might just have an old board lying around or a part kit, you never know do you.
 
Yes I read the thread on TAOS and Nick de-potted Markos Lentek and fitted Wimas to replace the ceramics and Oscons to replace the Tants
Said to sound excellent by all accounts. I may give it a go when I have time
At one of the NEBO events we tried quite a few different combinations or MC phono stages and MM + SUTs or head amps plus one of Nicks valve phono stages which are superb.
Marko and Ali Tait brought along quite a selection of nice gear and they all sounded great really just subtle variations of the same thing, I remember Marco talking about the Lentek but I dont think it was there on the day?.
My paradise Phono stage was also up there with the best of them but that,s another story :D

Incidentally the unmodified Lentek sound great with a simple MM phono
Here is mine with a cheap Chinese mm board which has a remarkable resemblance to the Bugle 2

. by Alan Towell, on Flickr
118 by Alan Towell, on Flickr
Hope you get the 323s working, maybee Martin Clark can assist :)
Alan
 
Yes I've seen your mm stage before Alan and I would have thought it was a Bugle if you hadn't said. It does like very much like one doesn't it.
Never been to a Nebo or heard the Paradise so maybe something for the future eh?

Anyway I'll have another mess with the 323 boards tomorrow and see I can't get some life out of them.
 
With a 10uf cap straight after Q3 and 100K to ground at the output the modified 323 cards are working well as a moving coil head amp into my valve phonostage. I need to do some more listening and comparing before deciding what to do next. I have a Stageline which might be more difficult to sell on because it's been modified somewhat. It has Avondale regs fitted and a modded external Avondale psu and I'm thinking that if it's not worth that much then I should use it for this project. I'm thinking that there are better ie lower noise replacements for Q1, Q2 and Q3?
 
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