Poor Man' s Tube Mic Pre Channel Strip (Eurochannel)

[chrispsound]

I have the 150k zobel network resistor with no capacitor on all my tube pre's with the JT-115K because thats what Jensen call for as far as a zobel is concerned.  I will try to bypass the input transformer run RMAA and see what happens.  I seem to remember someone saying that Edcor output transformers cause a little bump that starts in the 10kHz region.  Thanks, ChrisP.

 

[ruffrecords]

I have the 150k zobel network resistor with no capacitor on all my tube pre's with the JT-115K because thats what Jensen call for as far as a zobel is concerned.  I will try to bypass the input transformer run RMAA and see what happens.  I seem to remember someone saying that Edcor output transformers cause a little bump that starts in the 10kHz region.  Thanks, ChrisP.

It is most likely to be one of the transformers. The amp on its own is flat out to about 150KHz. How is the output transformer loaded?

Let me know what you find.

Cheers

Ian

 

[chrispsound]

The output transformer primary is the 4.7uf output cap and audio ground ( I do not use the 100k shunt resistor because I am using a transformer, right?) and secondary goes straight out to my xlr jack which goes out to my 10k input A/D converters.  Should I load the secondary with a resistor of some kind?  Thanks, ChrisP

 

[ruffrecords]

The output transformer primary is the 4.7uf output cap and audio ground ( I do not use the 100k shunt resistor because I am using a transformer, right?) and secondary goes straight out to my xlr jack which goes out to my 10k input A/D converters.  Should I load the secondary with a resistor of some kind?  Thanks, ChrisP

OK, understood. With a point to point set up you can get away without the 100K shunt resistor. You need it on the PCB version because the transformer is off board.

Some transformers' response depends on the load. You could try loading the output with 600 ohms and see if the response changes.

Cheers

Ian

 

[chrispsound]

I put a 560 ohm resistor across the secondary (because that's what I have) and got rid of my brain damaged potentiometer setup and the 10kHz bump went down below .5 dB, but is still there.  Is that the proper shunting resistor or is there a more standard one?  I will be ordering a 1k audio pot and could get resistors at the same time.  I am waiting for a 2:1 transformer from Edcor(it will be a while), maybe that will help flatten things out.  Even with the bump, this pre sounds great.  Don't rush, on my account, getting the component changes done for the strapped 12AX7, this thing is more than quiet enough (but I wont complain if its quieter).  I have to wire in a DI and I some how forgot to press in a turret so I can change components and strap the 12AX7 when I have the board out.  What is the output impedance of the 6922 in this setup?  Can't wait to be employed again so I can get a nice 2U case and custom front panel made up proper for it.  Thanks again Ian for all your help and prompt reply's. ChrisP

 

[ruffrecords]

I put a 560 ohm resistor across the secondary (because that's what I have) and got rid of my brain damaged potentiometer setup and the 10kHz bump went down below .5 dB, but is still there.  Is that the proper shunting resistor or is there a more standard one?

620 would be closer to 600 but the exact values does not matter. The point is you only got a small change in response by doing this and you should not need to  load the transformer with 600 ohms to make it work properly. I suspect the remaining bump comes from somewhere else - possibly the Jensen.

I will be ordering a 1k audio pot and could get resistors at the same time.  I am waiting for a 2:1 transformer from Edcor(it will be a while), maybe that will help flatten things out.  Even with the bump, this pre sounds great.  Don't rush, on my account, getting the component changes done for the strapped 12AX7, this thing is more than quiet enough (but I wont complain if its quieter). 

I probably won't get round to this until after Xmas now.

I have to wire in a DI and I some how forgot to press in a turret so I can change components and strap the 12AX7 when I have the board out.

I would recommend using the relay to switch in a DI input as it avoids having a length of cable connected to the transformer secondary when in mic pre mode.

What is the output impedance of the 6922 in this setup?

The 6922 SRPP stage has an intrinsic output impedance of around 1500 ohms. However, the NFB is arranged to be close to 20dB at all gain settings so the output  impedance gets reduced to around 150 ohms. By the time it gets through a 4:1 transformer it looks like less than 10 ohms. Even with the recommended 2:1 transformer it is less than 40 ohms.

Happy Christmas

Cheers

Ian

 

[chrispsound]

Removed the Jensen and went in bareback with RMAA and it still has that boost @ around 10kHz.  I went over everything but I will go over everything again with a fine tooth comb(3rd times a charm) while I wait for the Edcor.  I put a new, quality 100 nf film capacitor post zobel because the old one was used and abused and no change.  I already stole the Jensen for another project that is nearing completion so this pre will be on the back-burner for a while.  I am going to order  order a Cinemag or another Jensen/s input transformer for this project and others ASAP because Santa came early for me this year.  I am considering ordering a output transformer from Cinemag as well.  I have 2 mystery 1:10 input transformers that drop off around the same frequency that is being boosted, If I have time I will slap one in as a band aid for poops and giggles.  Is there a way to bypass the output transformer for diagnostic purposes.  Since the output impedance is approx 150 ohms can't I just go straight out off the output cap or will I need a shunting resistor of some kind.  Thanks, ChrisP

 

[ruffrecords]

Removed the Jensen and went in bareback with RMAA and it still has that boost @ around 10kHz.  I went over everything but I will go over everything again with a fine tooth comb(3rd times a charm) while I wait for the Edcor. 

You need to do 2 things. First check the frequency response with no transformers - you will need the 100K resistor from output to ground to make sure the output cap has a route to ground. Second check RMAA itself to make sure the rise is not in your sound card.

Cheers

Ian

 

[chrispsound]

The first thing I did (and now have done twice) was check the soundcard and cables with a loopback test and they were fine.  I disconnected both transformers and there is no longer that boost.  The problem now is I took the Jensen out for another pre that I am finishing up and dying to hear.  When I get a proper input transformer I will do the test with input trans and keep the output trans removed from the circuit and report back.  Again many thanks for your patience and help, ChrisP.

 

[ruffrecords]

Hi ChrisP,

OK, so we no it is not the tube amp - must be something to do with the transformers. I notice you only test up to 20KHz and it looks like the response might well keep on increasing above that. I have had a similar occurrence with a Sowter input transformer and it depended on the source impedance of the test oscillator. I found the response kept on rising up to a couple of dB at around 40KHz. If memory serves, this occurred when set to mic input where the oscillator source impedance is seen directly by the transformer. However, when used via the 20dB pad, which has an effective source impedance of 150 ohms the response  drooped by half a dB or so at 20KHz. I have not made a record of these tests so I need to repeat them to be sure I am not talking rubbish.

Cheers

Ian

 

[chrispsound]

I use a -40 db(approximate) u pad attenuator( 2- 10k resistors and a 150ohm shunting resistor) from my M-Audio 2626 line out ( 300 ohm output impedance balanced) for testing the preamps mic input.  But the high boost is still present when I use the on board pad or not.  The boost is still there with NO input transformer in circuit and output transformer left IN circuit.  The boost is gone without any transformers in the circuit.  I am pretty sure its the output transformer.  Could the low output impedance from the output tube, the output cap and the impedance of the 4:1 output transformer be causing some sort of bright boost?  Lassoharp also makes a statement regarding the 10k:600 edcor output transformer here:

I forgot to add, my output transformer is the Edcor XSM10K:600, which has a primary inductance of 40H.  If my math is correct, the -3db point for low frequency response is about 9Hz with a 3.4K load on the secondary (f = (60K || Rp || Rp)/(2*PI*40)).  Not too bad either!

You also have a 4.7uf & 1uF cap there to 'bump' somewhere ( ~ 11Hz?.)   

Don't forget to figure in the effects of the NFB on lowering output impedance on the output stage.  Even at at 3K I think you'll find the 10K:600 should sound pretty good.

That Edcor may naturally rise a bit on the upper octave.  Some have reported this in other threads.  I have heard it in some cases on the bench w/ or without a 600r terminating resistor.  This may work out to be a good thing.  Often times a 600r secondary can get fussy working into a higher Z load = fuzzy.  Very easy to wire in a switch for termination if needed.

When he says upper octave he means the 10kHz region?  At this point I don't know my behind from my elbow but I will keep at it, its probably something that I shouldn't be concerned about but just knowing its there bothers me.  "Ignorance is bliss".  Thanks, ChrisP

 

[ruffrecords]

When he says upper octave he means the 10kHz region?  At this point I don't know my behind from my elbow but I will keep at it, its probably something that I shouldn't be concerned about but just knowing its there bothers me.  "Ignorance is bliss".  Thanks, ChrisP

OK, understand about your test set up. As you say, the evidence points to the output transformer. I guess upper octave means 10KHz to 20KHz. A 0.5dB boost at 20KHz is not really something to worry about but if it extends to 2dB at 30 or 40KHz then you may want to tame it.

It is unlikely the amp output impedance and the capacitor would do anything at high frequencies - more likely to contribute to roll off at the low end. Boost in a transformer I would expect to be more to do with self resonance. Since there is more inductance in the primary and hence more turns and therefore capacitance I could see this might resonate somewhere above 20KHz. To alter it I guess you might try adding a resistor in parallel with the primary winding. Might be worth a try but I am not a transformer expert.

Happy Christmas

Cheers

Ian

 

[chrispsound]

I am still waiting for the 2:1 transformer (and a custom power transformer), Edcor sent it too the wrong address :'(  In the meantime I have been playing around with the Eurochanel circuit.  I took out the negative feedback and gain switch and instead of a 12AX7 I used a 12AU7 (strapped in parallel) in a common cathode amp configuration with a pot before your srpp output stage.  Running samples through it I hear slightly less mids but it seems to smooth out the highs nicely(perfect for my mic locker full of MXL's and CAD's).  Not even close to how clean your original circuit sounded.  THD still seems to be low.  My question is, do I have to optimize the SRPP stage since I took out negative feedback?  Without negative feedback the output impedance rises so does that mean I can use a 4:1 output transfomer without the same loss in headroom?  Thanks, ChrisP

 

[ruffrecords]

I am still waiting for the 2:1 transformer (and a custom power transformer), Edcor sent it too the wrong address :'(  In the meantime I have been playing around with the Eurochanel circuit.  I took out the negative feedback and gain switch and instead of a 12AX7 I used a 12AU7 (strapped in parallel) in a common cathode amp configuration with a pot before your srpp output stage.  Running samples through it I hear slightly less mids but it seems to smooth out the highs nicely(perfect for my mic locker full of MXL's and CAD's).  Not even close to how clean your original circuit sounded.  THD still seems to be low.  My question is, do I have to optimize the SRPP stage since I took out negative feedback?  Without negative feedback the output impedance rises so does that mean I can use a 4:1 output transfomer without the same loss in headroom?  Thanks, ChrisP

The ultimate drive capability of the SRPP output stage is determined by the idle current which you have not changed so I don't think there is much optimisation to do. You can raise the current a bit to improve the drive by changing the cathode resistors to 470 ohms. With this and a 2:1 transformer you can even drive 400 ohm headphones.

If you use a 4:1 transformer then you won't lose much in the way of headroom. It should still be able to do +22dBu into 600 ohms. The distortion will be higher without the NFB but at normal levels it should not be noticeable.

Cheers

Ian

 

[chrispsound]

I am truly amazed by the sound achieved with such a low low parts count of the 12AU7 common cathode + 6922 srpp.  Vocals don't seem to stick out in front so far but boy does it smooth stuff out.  I just gutted an older project and gave it its own home.  Is it possible to stick an ac coupled cathode follower such as the circuit found at the very bottom of this page http://www.valvewizard.co.uk/accf.html in front of your srpp output stage?  I would love to hear vocals through the many effects pedals I have access to, in this effects loop circuit.  Would a 1:10  mic input transformer bolt up to that valve wizard schematic?  Seems to me it would be better use of the 12AU7 rather than just strapping it.  Thanks, ChrisP

 

[dmp]

Hi Chrispsound,
Where do you get that drilled board you call 'universal prototype 2 bottle board'?
Looks very cool.
I'd be curious about a 12aY7

 

[chrispsound]

Watts tube audio, http://www.turretboards.com/ they also have the worlds best turrets to go with that board but they are not cheap so I try to use the eyelets in some spots to reduce cost.  I use my drill press to press the turrets with tools similar to what they sell at Watts.  When I got something going I like I buy FR4 from mcmaster carr and make my own turret board because it is a bit cheaper from mcmaster(although you do have to cut and drill it).  ChrisP

 

[ruffrecords]

I am truly amazed by the sound achieved with such a low low parts count of the 12AU7 common cathode + 6922 srpp.  Vocals don't seem to stick out in front so far but boy does it smooth stuff out.  I just gutted an older project and gave it its own home.  Is it possible to stick an ac coupled cathode follower such as the circuit found at the very bottom of this page http://www.valvewizard.co.uk/accf.html in front of your srpp output stage?  I would love to hear vocals through the many effects pedals I have access to, in this effects loop circuit.

You can but there is no need to as the SRPP has a high impedance input as it is.

Would a 1:10  mic input transformer bolt up to that valve wizard schematic?  Seems to me it would be better use of the 12AU7 rather than just strapping it.  Thanks, ChrisP

Again you could but there's not a lot of point. Strapping the 12AU7 is actually a good idea for the first stage of a mic pre because it lowers the noise by 3dB.

Cheers

Ian

 

[ruffrecords]

Is it possible to use just the mic pre section with a 12AX7LPS , 6922, Jensen 1:10 input transformer and  Edcor  10k/600 output trans.  If so then you would not have to include the 10uf filtering electrolytic between stages?  Would you ground the unused pins from the unused half of the 12AX7 or just strap them together.  Have you tested it in this arrangement?  I don't need a whole lot of gain and I love the design and parts count.  Thanks, ChrisP

When I first replied to this post I said you could strap the two halves of the 12AX7 together but you would need to change some component values. One advantage of this would be that noise should be lowered. Anyway, I finally got round to working out the component changes and simulating the updated circuit. I have attached the schematic so if you want to update your point to pint version you have the info. Basicaaly you only need to change three components.

The 12AX7 plate resistor becomes 75K
The 12AX7 cathode resistor changes to 235 ohms (two 470 ohms in parallel) and 33K respectively. I also took the opportunity to increase the idle current in the output stage which gives it a bit more drive capability. This is done by changing the two cathode resistors in the 6922 from 680 ohms to 470 ohms.

I am playing around with an idea for building one of these powered by a 12VAC 1amp wall wart. It makes the power supply universal and gets the mains transformer out of the box. Built into a 2U rack case this would replace the mic pre demo unit that was stolen last year.

Cheers

Ian

 

[chrispsound]

Thanks Ian, I will implement those component changes and try this badboy out.  This pre is quiet with the power supply in the box and AC on the heaters, I can't imagine how quiet it would be with your external setup  I have been playing around with your srpp output and I like it behind the 2 triode common cathode input stage with the feedback loop (ac coupled) going back to the cathode of the second triode stage(like the NYD MILA), a EuroMILA.  Thanks for getting back to this one, I truly enjoy building all your projects.  ChrisP