You can also use OPA604, OPA627, OPA827, ADA 4627, AD711 etc.
All Fet opamps with low offset.
All Fet opamps with low offset.
With my bench PSU, I have correct input voltages +-24v and Right channel is much better:
Output offset is now between .2 and .1 V oscilating
Input offset -1.8mV steady
The Left channel is almost as before
Output offset goes from 14V to -12 very fast
Input offset oscilates between 5 and 8 volts
When I connect the Left channel I can see flicker on all the leds... the oscilation extends to both channels.
If I leave only the Right channel on, everything is more stable.
Output offset is now between .2 and .1 V oscilating
Input offset -1.8mV steady
The Left channel is almost as before
Output offset goes from 14V to -12 very fast
Input offset oscilates between 5 and 8 volts
When I connect the Left channel I can see flicker on all the leds... the oscilation extends to both channels.
If I leave only the Right channel on, everything is more stable.
The AD825 is only specified to +-15V, so you need 2 Leds more or use 4 Leds that drop 9V. 2 blue and 2 red drop 9V. 3 green and 1 blue would just make it.
1,8mV input offset is a bit much. Is that with only the loading resistors or the cartridge connected ? If you do not want to put that DC into your cartridge use a resistor parallel to the input load that is similar to your cartridge impedance. What cartridge do you use ?
Anyway, one channel works better. I would compare now all the voltages. First compare the voltages over the emitter resistors of the mirrors.
Anyway, one channel works better. I would compare now all the voltages. First compare the voltages over the emitter resistors of the mirrors.
On switch off, the input offset goes from -1.8mV to -40mV before going to zero
I will test it with a Denon DL103.
I have been testing without cart, only with a 450r in paralel with the input.
I will test it with a Denon DL103.
I have been testing without cart, only with a 450r in paralel with the input.
With 40 ohms as load, input offset goes down to -0.1mV steady 🙂
Output offset oscilates between .2 and .08V
Output offset oscilates between .2 and .08V
A few comments/suggestions regarding the prototype PCB layout:
Standoff mounting positions should be holes. (I noticed they are on the silk only on the beta tester boards)
I much prefer part designators rather than values on a PCB. The values might change, but the designators will stay constant. It also makes debugging the circuit much easier, if needed. Now which BC327 I am looking for again? 😉
Add a separate ground connected pad for experimental connection to chassis ground, ground post, etc.
I'm not sure about the location of the servo circuitry. Would it not be "better" to move the POS and NEG phases of the amplification circuit and RIAA components closer together and have the servo located closer to the output? Or is the current location better because of close proximity to the input?
Standoff mounting positions should be holes. (I noticed they are on the silk only on the beta tester boards)
I much prefer part designators rather than values on a PCB. The values might change, but the designators will stay constant. It also makes debugging the circuit much easier, if needed. Now which BC327 I am looking for again? 😉
Add a separate ground connected pad for experimental connection to chassis ground, ground post, etc.
I'm not sure about the location of the servo circuitry. Would it not be "better" to move the POS and NEG phases of the amplification circuit and RIAA components closer together and have the servo located closer to the output? Or is the current location better because of close proximity to the input?
All my measurements are with external psu connected in the central point of the board... the inboard psu is still connected so the leds light up... I believe it should work better if I disabled the inboard shunts... but how ?
Does anyone have any experience with these capacitors for RIAA use?
AMTRANS Products Information AMCH
AMTRANS Products Information AMCH
ungie, Hesener should talk to you. He is our layout expert.
We use BC327-40 and BC337-40. This are the ones with the highest Hfe. So far we tested Doitec and Fairchild.
I have Amtrans in PPS. They have a good reputation but i did not try them so far.
RRicardo, now is cutting time. Cut over the 100uF caps. To remove the ground is harder.
We use BC327-40 and BC337-40. This are the ones with the highest Hfe. So far we tested Doitec and Fairchild.
I have Amtrans in PPS. They have a good reputation but i did not try them so far.
RRicardo, now is cutting time. Cut over the 100uF caps. To remove the ground is harder.
hi ricardo, sorry to hear you are having trouble....
to disconnect the shunts, just take out one of the LEDs from the strings, this disables the shunt. you should also desolder the power resistor from the current source just to be safe.
have you tried with a scope to see if this oscillates? you can connect the ground clip to the tip of the probe and see if you get a 10-20mhz signal. in that case, try the ground plane shunt. i have seen large output variations that looked like DC but were caused by the parasitic high frequency oscillations.
good luck
I connected output to input GND and now I am reading input offset (Left) .6v instead of 9v
Maybe I should try the GND plane shunt.
As I have one fully functional channel, I will focus on the other one only (Left)
With shunts disabled, there is no flicker on the opamp leds... good.
With 40r load I have 40mV offset in the input...
In the output I read 13.6v offset very stable now.
All test points read ok
tp1 24.1v
tp8 23.7v
tp7 23.7v
tp6 22.6v
tp5 22.6v
tp3 13.3v
tp4 -12.5v
tp9 -22.8v
tp10 -22.8v
tp11 -23.7v
tp12 -23.6v
tp2 -24v
I read 13.9v on the bases of Q35 Q38 (This is wrong)
So the riaa circuit reads also 13.9V
Replaced Q28 but the issue persists....
If Q15 is shorted, can it pass tp3 (13.3v) voltage to R47 and R51 (22ohm each) ?
With shunts disabled, there is no flicker on the opamp leds... good.
With 40r load I have 40mV offset in the input...
In the output I read 13.6v offset very stable now.
All test points read ok
tp1 24.1v
tp8 23.7v
tp7 23.7v
tp6 22.6v
tp5 22.6v
tp3 13.3v
tp4 -12.5v
tp9 -22.8v
tp10 -22.8v
tp11 -23.7v
tp12 -23.6v
tp2 -24v
I read 13.9v on the bases of Q35 Q38 (This is wrong)
So the riaa circuit reads also 13.9V
Replaced Q28 but the issue persists....
If Q15 is shorted, can it pass tp3 (13.3v) voltage to R47 and R51 (22ohm each) ?
Ricardo, i go to bed now. The only tip i can give you at this moment : make sure that you have not swapped NPN and PNP somewhere.
There is one way to check without desoldering or measuring. Bend the leads somewhat so that you can see what transistor you have. You can do that best when the light comes from the side. Like people do that repair old paintings.