Doug Self's New Preamp Construction and Pictures

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delange how is the Moving-Coil/Moving Magnet board with your tests?

I didn't test it with that board because I didn't know about its existence.
I carefully used a signal generator with a voltage divider to test. As soon as this setup confirmed that my phono preamp was working I hooked it up live.

I will probably build such a test board now. It is a better way of testing phono preamps.
 
Delange - Yes, I've been using the phono stage, with an Ortofon 2M Black MM cartridge, modified Rega RB250 arm, and my Inspire Apollo deck. For MM, it is pretty well silent as far as hum or hiss is concerned. Playing music, it is clean and detailed with a wide sound-stage - very nice! I can't comment on performance with MC as I haven't used it.

I did consider the 2M Black myself. Some people say that the 2M produces some surface noise which is quite noticeable between tracks. During the shoot out session I had between the AT-OC9/III and the 2M Black I didn't really notice that. What is your experience with surface noise?

In the end I chose the AT cart over the 2M because I felt that voices had a bit more body with the AT. To my ears both carts are quite close to each other (this was on the system of the hifi dealer).
I agree with you on the sound signature of the preamp; it does sound very very nice.


I did some more testing with the phono preamp:

* When the turntable is disconnected then the preamp is silent (volume turned up all the way).

* With the turntable connected there is a bit of hiss and hum (volume turned up all the way).

* With the turntable connected, but the headshell removed there is no more hiss but only a bit of hum (again volume turned up all the way). I guess this is to be expected with 60 dB of gain and an "antenna" (= the cables unterminated) attached to the preamp. This test would also suggest that the hiss is introduced by the cart. I would have hoped that with the cart connected it would remove the hum due to the fact that the cables are now terminated by the cart.

I tried to connect a separate ground wire to several places on the turntable (arm, headshell, turntable body, etc) just to see if there is any difference in hum when doing that: no difference at all. This leads me to conclude that at least the grounding wire of the turntable is not the cause.

Even though the hum is quite small, it is noticeable at higher listening levels. I find that quite annoying.
Maybe I'm trying to chase ghosts here; amplifying a tiny signal from an MC cart is a tricky job. With high gain settings it is easy to pickup unwanted signals.
Anyway, I would love to hear some experiences from other users with an MC cart in this regard.
 
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* With the turntable connected, but the headshell removed there is no more hiss but only a bit of hum (again volume turned up all the way). I guess this is to be expected with 60 dB of gain and an "antenna" (= the cables unterminated) attached to the preamp. This test would also suggest that the hiss is introduced by the cart. I would have hoped that with the cart connected it would remove the hum due to the fact that the cables are now terminated by the cart.

For completeness I must add that the hum is also way lesser with the headshell removed. Both the hum and hiss seem to be introduced by connecting the cart to the preamp.
 
For your MC stage and AT-OC9/III, have you set the input loading up? The manufacturers' recommended input R of 100 ohm is catered for by R2 / R50 of the input stage. Try adding a 4.7nF into C1 and C28, this should help with surface noise and radio interference without reducing frequency response.
 
For your MC stage and AT-OC9/III, have you set the input loading up? The manufacturers' recommended input R of 100 ohm is catered for by R2 / R50 of the input stage.

AT recommends 100 ohms loading and this is how the preamp is setup by default (R2 and R50).


Try adding a 4.7nF into C1 and C28, this should help with surface noise and radio interference without reducing frequency response.

Just to be clear, the AT cart doesn't produce any surface noise.
I'll give the 4.7nF cap a try.
Due to the very low output impedance of MC carts, the C loading of such carts shouldn't change their behavior much. But maybe it helps against hum / RF interference.
 
Update:

* I added a 4.7nF cap on the MC input and that reduces the hiss quite a bit (this is with the volume turned up all the way). The caps do not affect signal quality, which was to be expected.
The caps do not change the hum behavior.

I did some further testing:

* Trying to figure out if the hum is due to grounding, I did various grounding tests as explained in the previous posts. Non of those tests change the hum behavior. It is safe to say that I'm not having a grounding issue.
But the above let me to remove the headshell while the volume is turned up all the way. To my surprise, this eliminates the hum. Due to that I measured all cable connections (turntable RCA and turntable ground). All good.

Since I have my old MM cart mounted in a different headshell, I now mounted my MM cart to check things: no hum (okay, a very very very very tiny bit) with the volume turned up all the way (phono preamp still in MC mode).

So, the hum is due to the AT cart? I'm starting to think this is due to some impedance mismatch between the cart and the phono preamp. But I'm all out of ideas at this point.


* So while I was playing around with my MM cart, I decide to readjust my turntable and fitted the MM cart again. Connected the MM cart to the MM input and set the phono preamp to +45 dB overall gain. This plays very well with no hum in the background. Even with the volume turned up all the way, the hum is very very very tiny. It is not noticeable between tracks as it was with the AT cart.

* While I was playing around with all of this, I re-installed my old phono preamp. Funny thing here is that there is absolutely no hum whatsoever with the volume turned up all the way up. There is however a ton of noise (hiss). Compared to my old phono preamp, the Elktor preamp is extremely quite noise wise but it has a very very very tiny bit of hum (volume all the way up) in MM mode with my current MM cart.

Well, I honestly am getting tired of this annoying issue. Should I just go back to MM carts? I could sell the AT cart and buy an new MM cart and be done with this. It's a shame because the AT cart sounds really great.
 
Update:
I did some further testing:

* Trying to figure out if the hum is due to grounding, I did various grounding tests as explained in the previous posts. Non of those tests change the hum behavior. It is safe to say that I'm not having a grounding issue.
But the above let me to remove the headshell while the volume is turned up all the way. To my surprise, this eliminates the hum. Due to that I measured all cable connections (turntable RCA and turntable ground). All good.

Since I have my old MM cart mounted in a different headshell, I now mounted my MM cart to check things: no hum (okay, a very very very very tiny bit) with the volume turned up all the way (phono preamp still in MC mode).

So, the hum is due to the AT cart? I'm starting to think this is due to some impedance mismatch between the cart and the phono preamp. But I'm all out of ideas at this point.
From what you say about only a tiny bit of hum with the MM cart/headshell with phono still in mc mode I would say your problem is grounding of the headshell that you have the MC cart fitted to (MC cart itself is fine). Try temporarily grounding the head shell itself and see what happens with the hum. That may help isolate where the grounding issue is.
 
From what you say about only a tiny bit of hum with the MM cart/headshell with phono still in mc mode I would say your problem is grounding of the headshell that you have the MC cart fitted to (MC cart itself is fine). Try temporarily grounding the head shell itself and see what happens with the hum. That may help isolate where the grounding issue is.

Thanks Geoff, but I already tried that in my previous tests. Did not change anything. Today I mounted my MC cart in the headshell where my MM cart was mounted. This would excluded the different headshell from the equitation. No change here either...

Do you hear any noise and / or hum from your Elektor phono preamp when you turn the volume all the up (without playing music of course)?
 
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Not that it'll effect the hum ("The high inductance of an MM cartridge makes low current noise important), but you did read about the value options of C1, C28 (0-10nF) and R1, R49 (10R-1K)?
Also:-
C8, C35 (0-330pF) and R13, R61 (lowest recommended is 220K).

Perhaps reading through Part 2 May 2012, page 28 & 29 might help?
 
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Not that it'll effect the hum ("The high inductance of an MM cartridge makes low current noise important), but you did read about the value options of C1, C28 (0-10nF) and R1, R49 (10R-1K)?
Also:-
C8, C35 (0-330pF) and R13, R61 (lowest recommended is 220K).

Perhaps reading through Part 2 May 2012, page 28 & 29 might help?

Yes, but as the article states, this is particularly so for MM carts.
I'm having hum issues with my MC cart. The preamp behaves well with my MM cart.

Adding a 4.7nF cap did reduce the hiss on the MC input. Although Audio Technica (AT-OC9/III cart) does not advice to do that.
 
Delange, although you've probably read it, in Pt2 it says: -
"Component positions R1 and C1 are provided so the cartridge loading can be modified.
This has only a marginal effect on MC cartridge response in most cases because the cartridge impedance is so low. However, if you want to experiment then the appropriate
range for R1 is 10 Ω- 1 kΩ, and for C1 0–10nF."
Nothing about hum though, sorry......
You have one meaty cartridge!
AT-OC9/III - Moving Coil Cartridge | Audio-Technica
 
Another update:

The good thing about all the time I've put into this and all the things I've checked and measured is that I can eliminate a lot of stuff. I'm now focusing on internal grounding (inside the preamp enclosure) and possibly magnetic interference of the transformer. I thought I did gave this a lot of attention already but maybe not enough. Since the beginning I had some doubts about the way the PCB is laid out (in terms of grounding). I need to check a few things first before going in to deep about this but this is were my attention is directed to right now. I'll keep you posted.

Geoff, I would still be interested to hear about what kind of noise / hum you experience when you turn up the volume completely (without playing music of course).
 
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Update... The DS 2012 preamp is finished! It's been a long haul, but I'm very happy with the end result. I went with a little different look with the black knobs on a silver FP. I think it looks kind of 'smart' personally. I wasn't quite sure which would look the best, silver, or black. I do have a set of silver knobs should I want a more conventional look down the road. Finding just the right, proportioned sized knobs proved very interesting to say the least. You just can't tell what they will look like unless you try some physically. So now I have bin full of different size sets of knobs - some larger, some smaller. These are the best looking ones in my opinion.

If I can give a shameless plug for Front Panel Express. As I mentioned in an earlier post, this was my first time trying them and I can say I was pleasantly surprised by the high quality of their service. Their software is fairly easy to use and for one-off panels their price is fair. I couldn't have even come close with a drill press and press-on lettering! :p I highly recommend their services.

Thanks again to Doug Self for designing and publishing this fine preamp project and Elektor for making available the PCBs. This pre is obviously a real contender, and certainly eclipses, in audio performance, the rest of my non-esoteric gear. But, I wanted this DIY project as a challenge and I'm glad I took the many months to see it through to completion. I'm very satisfied with the result. :)

Rick

well done very impressed, great detail and craftsman ship ..I might make one now
 
A question to put some light on.
In Elektor Pt1, line/tone/volume pcb, page 21 it says:-
'Do not forget to fit JP1 for the ground through connection, a similar jumper is present on the MC/MM board.
This allows you to determine empirically which ground connection works best. When the wiring is complete one jumper has to be fitted, or both'.

Have constructors needed to use one or both ground connections?
 
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The grounding certainly needs some attention.
The phono preamp PCB is very sensitive to interference pickup. I did some testing by moving my toroidal transformer around. Toroidal transformers are known to have far less magnetic stray fields. But the preamp picks up hum even when the transformer is some 25 centimeters away!

I'm suspecting this is due to the way grounding is laid out on the PCB. The screening is also use to connect various ground connections for components. This is not a good idea. Elektor should have implemented star grounding on the PCB. The shield should only be shield and not to be used to connect ground to different components.

I still need some further testing and experimenting on the matter.
Will keep you posted.
 
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