HPF Solution... help needed

[gyraf]

Now I just need to get the HPF back in business.

You have 30K input grid resistor at EF86 (yes, I know it's a trafo termination, but anyway).

47nF between transformer secondary and 30K would give you 112Hz HPF
100nF would give you 53Hz HPF

Jakob E.

 

[ruffrecords]

Now I just need to get the HPF back in business.

You have 30K input grid resistor at EF86 (yes, I know it's a trafo termination, but anyway).

47nF between transformer secondary and 30K would give you 112Hz HPF
100nF would give you 53Hz HPF

Jakob E.

I am puzzled why the first stage grid resistor is as low as 30K. Usually it is much higher to avoid loading the transformer and wasting precious low level signal. Which input transformer are you using?

Cheers

Ian

 

[johnheath]

Jakob: Interesting! Is that a good Place for a HPF? I have not seen all schematics in the World but I have not seen a HPF right after the input transformer... I have no opinion on this just asking.

Ian: I had a sowter 4935 left so I used it in this case... is the 30k wrong?

Regards

/John

 

[ruffrecords]

Jakob: Interesting! Is that a good Place for a HPF? I have not seen all schematics in the World but I have not seen a HPF right after the input transformer... I have no opinion on this just asking.

Ian: I had a sowter 4935 left so I used it in this case... is the 30k wrong?

Regards

/John

The  Sowter 4935 is a 1:7 ratio transformer so the primary to secondary impedance ratio is 1:49. As a rule in a mic pre you aim for the input to 'look like' about 1K5. To do this the secondary resistor needs to be 49 times this; in other words 49 x 1K5  which is about 75K. With a 30K secondary resistor of 30K the input 'looks like' 600 ohms. With most modern mics being 200 ohms or less and condenser being around 50 ohms then you will probably be OK with 30K.

Cheers

Ian

 

[abbey road d enfer]

Jakob: Interesting! Is that a good Place for a HPF? I have not seen all schematics in the World but I have not seen a HPF right after the input transformer... I have no opinion on this just asking.

Ian: I had a sowter 4935 left so I used it in this case... is the 30k wrong?

Regards

/John

Seems you didn't read my previous post: "I think you could insert a cap between the input xfmr's secondary and the grid resistor of the EF86. That would be "outside the NFB loop" and prevent any possible overload of the 1st stage."
30k is the recommended load for 4935; reflected input Z is about 600r. I think you could probably go to 60k (or 62), for an input Z of 1200r that would be more in line with current practice. I don't think the possible HF ringing would be terribly disgracious.

 

[johnheath]

Thanks Ian

I did not know this, but their data sheet mentioned the 30k load when measuring frequency response and THD so I Went for it.

If I understand you correctly you still suggest me to change the resistor to a 75k? It is a 30 seconds job

Best regards

/John

 

[johnheath]

I am sorry Abbey:

Of course I read your answer but to be honest I did not really understand in what way and what value I should use... and it was really late here in sweden... no excuse I suppose though

Did you also mean that the HPF could be used right after the transformer secondary? If two experts say that it is surely something to consider for me. I have just adjusted some voltages according to PRR's suggestions .

Regards

/John

 

[johnheath]

Problem solved!

Thanks all for very much valuable information.

PRR: Voltages on second stage, NFB and impedance calculations.

Jakob and Abbey: For the "new to me" possible placement of the HPF and NFB.

Ian and Abbey: for the discussion of the input transformer and load resistor value and again the NFB.

To mention a few 


Best regards

/John

 

[ruffrecords]

Thanks Ian

I did not know this, but their data sheet mentioned the 30k load when measuring frequency response and THD so I Went for it.

If I understand you correctly you still suggest me to change the resistor to a 75k? It is a 30 seconds job

Best regards

/John

It can be difficult to read transformer data sheets because they tend to be full of inconsistencies. For example, I use a Sowter 10:1 transformer, the 9610. Its data sheet also has  the same measurements made with a  200 ohm source 30K load (insertion loss and frequency response), but a 30K secondary load will be reflected to the primary as 300 ohms. With these transformers I use a 150K secondary load. which is reflected to the primary as 1K5. I have measured the response with a 150 ohm source and it is flat. With a 50 ohm source, like a condenser mic, you get a bump above 20KHz. Then again, distortion is measured with a 40 ohm source.

So it is a bit of an engineering compromise all round. The difference between a 30K and 75K in your case will be small. a fraction of a dB improvement in signal level. A lot depends on your mics and the loading they need/prefer. If you have some vintage mics with an output impedance approaching 600 ohms then the 75K is definitely worth it. If you mainly have condenser mics then leave it at 30K.

Cheers

Ian

 

[johnheath]

Interesting stuff... I use mainly condenser mics when recording so I might be well with the 30k. 

Regards

/John