JLH phono stage problems

[Marra]

I have just built the JLH discrete phono stage .All power supply voltages are ok and dc offsets on MC settings are negligible but when set to MM I get readings on the inputs of 70.3mv on one channel and nearly double that on the other.
My electronics knowledge is minimal so any advice would be much appreciated.

 

[burbeck]

hi which JL Hood phono stage have you got? there are quite a few discreat versions that have been published, maby a circuit would help too

 

[Marra]

Hi Bob;my jlh phono is I believe the latest version;pcb 1450 and has the MC input directly coupled but MM connected through a 100uf capacitor bypassed by a polypropylene cap. The input transistors are BD438/BD437 TO126 devices. I think the design dates from the early 90's. I built a MC version of this many years ago which worked brilliantly. I have gone back to using my London/Decca gold hence the nead for a MM stage.
Regards Keith

 

[burbeck]

hi Keith
are we talking about this one K1450 (3/4 way down page)?

http://www.angelfire.com/sd/paulkemble/sound3.html

there are switches to set the amp to MM or MC, the input one alters the input impeadance MM = 47K and MC 100R. the other part of the switch sets the gain of the stage.

i would say if the moving coil settings are ok then i would check the values of the feedback resistors (150R and 12R) and the switched in put resisors (100 and 47K) it is very easy to put an incorrect value part, the colours somtimes get confusing.

also i would check wheather the switches are in fact working as they should.
i dont know what kind of switch that Hart used on the pcbs dil types maby?

give the whole thing a check for incorrect values especialy around the areas noted.
hope this helps, if not get back to me, we will get it up and running

 

[Marra]

Hi Bob; yes thats the one.
I've removed the pcb from my preamp and will check the things you suggest over the next couple of days and see if anything comes to light.
Thanks Keith

 

[burbeck]

hi Keith
i have built a few of these although not on the hart pcbs, i use one myself and i like the sound and the versitility to easy adapt to a wide range of carts
let me know how you get on

 

[Marra]

Hi Bob;
I had intended to build this JLH as a stand alone MM unit to go with the MC version I built some time ago. I am at the moment attempting turn the MC version into an MM/MC to fit inside my TVC preamp; having failed to sort the MM version. I know the MC version works so by adding afew components and the changedver switch[chassis mounted gold on/on type] I will have afully working unit.
Regards Keith

 

[godfrey]

... when set to MM I get readings on the inputs of 70.3mv on one channel and nearly double that on the other.

Looking at the circuit and the datasheets for the input transistors, it seems that nothing is wrong here - a bit of DC offset is to be expected at the "direct" input.

The input transistors each run at a little over 1 mA and have a current gain of typically a little over 100, so the base currents will typically be about 10uA.

Any mismatch in the base currents will flow through the 47k resistor, creating a DC offset at the input. e.g. a 2uA mismatch in base currents would give about 100mV offset.

IMHO it could be a lot worse - there's a huge difference between min, max and typical current gains.

btw; the much lower offset when switched to MC is because a much lower value resistor is connected across the 47K.

Similarly, when a cartridge is connected, the offset voltage with the MM setting should be much lower as the resistance of the cartridge is much lower than 47K.

It should be fine (assuming everything else is OK) - a couple of uA of DC through the cartridge won't hurt anything.

 

[Marra]

I built A MC version of this phono amp a few years ago; it worked perfectly with no problems. When I decided to start using my Decca gold cartridge again I thought I would build a MM version. I was confused by the highish dc offsets on its inputs[lack of knowledge here]. To cut along story short I then decided to build it as described in the original article and mount it in my TVC preamp. When fully wired in I found I had 0.255v and 0.334v respectively on MM inputs. MC readings were less than 1mv. So at the moment I am adding the MM components to the original MC board and will see what transpires.

 

[Marra]

Hi Bob and Godfrey. I now have a fully working JLH making beautiful music; detailed; dynamic perhaps a little narrow in the soundstage[ compared to my valve preamp]. Whether this will improve over time we will see; it has only been running for a few hours. Back to some listening.