E88C preamp ,comments critisisms required

[Tubetec]

Heres a schematic of a preamp Ive put together lately , it seems to do what I need it to do quite well.With lower level input voltages it seems happy driving mic inputs above 1.2k ohm ,At higher levels punches out plenty of volts into 10 kohm. The feedback arrangement works ok ,but the feedback/gain range does vary depending on whats hooked to the input .Ive also put in an A Nimh cell in series with the cathode ,Im quite happy with how this worked out ,its a little quirkey in that over time the cathode grid voltage varies over a small range as the cell charges ,but as its only around 12 ma of current flowing it takes weeks of use to charge the 4000mah cell .The output transformer matches around 2.5 kohm to 600ohm .
Id be really greatfull if Ian (ruffr) and Prr could cast a brasseye over this ,and suggest any  improvements that spring to mind.

 

[Tubetec]

I subequently changed the value of the feedback pot to 100k ,gives a bit more gain range .

 

[gyraf]

Output winding needs to refer to ground, or feedback is void..

 

[Tubetec]

Cheers Gyraf ,
Seems to work for me as is ,Im able to vary gain between around 6-20 db depending on what source im driving it with

 

[Tubetec]

This unit is always feeding a balanced input ,either mic or line

 

[abbey road d enfer]

Seems to work for me as is ,Im able to vary gain between around 6-20 db depending on what source im driving it with

Maybe, but it's serendipitous. If you connected it to a balanced receiver, the gain control would not work. You should revise your connections in order to make it work in all cases. That may just consist in making a permanent connection between on of teh secondary's wires to ground.

 

[Heikki]

If a compete redesign is out of the question, I would just take the negative feedback from plate to grid and add some series resistance to the grid to make the gain predictable.

 

[Tubetec]

Thanks Abbeyroad and Heikki,
I guess I could do a switched feedback arrangement from grid to plate with a padding resistor on the input for more predictable performance  impedence wise .
At any rate when feeding out to a balanced input the feedback gain control does work .
Ive tried feeding the input with a stepup (1.2k-25kohm) transformer,when I allow the grid resistor and pot to be the terminating impedence of the secondary of the input transformer I get loads of gain control ,when i terminate the input transformer with the correct load (approx 50 kohm ) I get alot less feedback/gain variation.

 

[abbey road d enfer]

Ive tried feeding the input with a stepup (1.2k-25kohm) transformer,when I allow the grid resistor and pot to be the terminating impedence of the secondary of the input transformer I get loads of gain control ,when i terminate the input transformer with the correct load (approx 50 kohm ) I get alot less feedback/gain variation.

This gain control works by applying NFB current, which changes the input impedance of teh overall circuit. That makes this gain control dependant on the source impedance. Clearly less active with low impedance sources.

 

[PRR]

> Prr could cast a brasseye over this ,and suggest any  improvements that spring to mind.

Get some light on the paper. Re-size the image smaller than a 64-channel console; the base plan is hardly bigger than a matchbook.

Other than that- it should pass signal. The NFB will not do anything as-drawn for two reasons.

But if you are happy, that's what matters.

> weeks of use to charge the 4000mah cell

600mAh NiCads are still readily available for garden lights. A much smaller NiCad used to be common for PC mobo CMOS backup. A common Carbon-Zinc AAA cell will work here. Dunno why you need a C cell.

 

[ruffrecords]

It is definitely a quirky little circuit but as PRR says it should pass signal. My first question would be what is meant to be used for?

Cheers

Ian

 

[Tubetec]

Appologies for the shoddy camera work !
Ok first the battery, I did initially try an AAA and AA cell in this unit ,both work fine ,but as the battery charges up the bias changes ,marginally . The idea of the larger capacity cell was that the rate of change of voltage of the cell is much smaller ,so keeping things a bit more stable. I intentionally discharge the battery to 1.2 volts to begin with ,and gradually, as it charges this rises to around 1,4 volts. Anode volts dont vary much between 1.2 and 1.4volts. Yes I have heard that Ni-cd maybe be superior in such an application, it was just I had Nimh's to hand .
I guess yes the feedback arrangement is dependant on outside factors ,not the most ideal setup . Maybe I have the feedback connection drawn on the wrong phase of the output transformer for it to work.

In terms of the application of this circuit ,a bit of an all rounder ,either direct box for highish  impedence sources ,or in conjunction with an input transformer and suitable matching resistance  for mic input .
I also have a direct out jack from the anode(post cap/pre transformer) which allows unbalanced connection( not shown on the drawing)
The output transformer is gapped ,and originally from a funky 1960's Philips  600 ohm speaker amp, driven with an anode loaded EL84, I decided Id give it a try RC coupling to remove DC from the primary.


Nothings set in stone  ,its such a simple circuit it can be re-jigged easily .
All sugestions graciously considered ,and thank you all kindly.

 

[Tubetec]

Generally  ,this unit will drive line inputs ,fed by a tube condenser mic ,ocassionally it might get used with a dynamic  and into mic input of a few kohms .