Mic Fet booster with stepped HPF and line drive

[mediatechnology]

Parts Express stocks Dayton axial polypropylene in large values for crossovers and they're an excellent value and measure well but any large film cap tends to be microphonic.

[AnalogJoe]

Electrolytics distort even when a small signal voltage drop is impressed across them, which makes them perfectly ok as decoupling caps if you place the cut-off frequency so low that the voltage drop across the cap is insignificant at audible frequencies, but if you use it as a filter with a higher cut-off frequency, they will have a significant voltage drop across them at audible frequencies.

I agree but don't you mean to say "coupling" (DC-blocking) rather than "decoupling?" (supply bypass)



I like the first bootstrapped FET input circuit better as it uses small value films.

Sorry, yes, I meant coupling capacitors. English is not my native tongue and I sometimes mess things up.

The bootstrapped FET does seem like a better choice.

[Tubetec]

Ive never been happy with the sound I get from standard soundcard mic input to A/D convertor/monitor out .
Without any way to manage the bass hump its unusable for close miked sources ,

There is a lot to like about the Sennheiser circuit , its elegant in its simplicity and as the years go on you dont have to worry much about the coupling caps slipping in value . The five HPF positions provided by the TFE/Canford gets me very close to where I want to be sonically without the need for more complex eq ,
As the frequency range tests show the almost 90khz upper bandwidth limit of the SSL2 is unimpacted with the circuit in place ,
FreE-Q seems like an apt name for it .

I hope you guys have the chance to check out one or other variations on this theme,to suit your own taste ,

I have to add the original inspiration for a passive HPF/proximity filter at the front end came from P.J.Baxanadalls 1950's high quality pre amp design , as I mentioned already I have a CF based LDC tube mic with a 12 position passive HPF at the output , matching into hiz unbalanced , the results are nothing short of superb , were not talking 'coloured' or 'fuzzy around the edges' or the 'warm' tube sound , its absolutely pristine , upto the highest SPL's you can throw at it ,
The next step is to try the CF/HPF/buffer in an impedence balanced fashion , signal output of course remains signal ended , but we get the benefit of any cable induced noise being reduced signifigantly .
An LDC/CF mic can output 20 volts peak on loud sound sources , the Sennheiser MZA-14 buffer is limited to around 700mV before distortion rises to an unacceptable level , thats likely to be perfect for a good quality 48V powered condenser .
Attenuation of the CF could be used , but preferably not as it will spoil the low output impedence , if attenuation is required , its better done after the buffer, likewise with series/parrallel output transformer secondary being used to alter the gain structure and drive impedences .


What happens if we want to power not only the buffer and mic polarisation but run a tube HT supply off the same rail ?
Transistorised class A buffer with 125v-150v supply ? its well beyond even the 2N3055's voltage rating ,a high voltage mosfet seems more appropriate .

Id also like to try out one of the Klark Technik dynamic mic boosters ahead of the HPF ,

[Tubetec]

https://groupdiy.com/threads/cloudlifte ... 014/page-2
Some helpful hints here , I wonder did Abbey ever get around to re-doing his his fet boost circuit in a more flexible format ,

[mediatechnology]

Sorry, yes, I meant coupling capacitors. English is not my native tongue and I sometimes mess things up.

Your English is excellent - I've always thought it was your first language.

[Tubetec]

I found a few Mosfets circuit of interest here ,
https://www.tubecad.com/index_files/page0021.htm
Maybe the choke loaded source follower but using a transformer on the output .
The transformer at the input isnt much use .

[AnalogJoe]

Sorry, yes, I meant coupling capacitors. English is not my native tongue and I sometimes mess things up.

Your English is excellent - I've always thought it was your first language.

Thank you for your kind words.

[AnalogJoe]

I found a few Mosfets circuit of interest here ,
https://www.tubecad.com/index_files/page0021.htm
Maybe the choke loaded source follower but using a transformer on the output .
The transformer at the input isnt much use .

Why do you need a transformer in a tube push-pull fashion? Compared with tubes, a big advantage of having N and P channel devices is that you don't need a transformer, there are no equivalent "P channel" tubes.

Chokes with solid cores are also non-linear, which makes them distort.

The circuits you posted are very interesting and look fun, but I believe you can achieve better results with less exotic and conventional circuits that do not require inductive elements.

[Tubetec]

Some interesting stuff about FETs here ,
https://groupdiy.com/threads/comparison ... 559/page-6 ,

Mosfet Buffer.JPG

Its something a bit like that I had in mind for the output stage , but using step up transformer in place of the choke ,
output being taken either direct or via the transformer secondaries .

It seems pretty clear now from what I've read , with transformer/choke coupled output , there no need for a high voltage supply or Mosfets ,

[Tubetec]

Plenty of down sides to transformers , I know that ,
It does change the overload characteristic of an amp , having a transformer with current flowing in it and hence stored electromagnetic energy in the core ,which it can use on the peaks , like a back EMF.
Now of course we know transformers dont tend to distort well with heavy bass content , but on short term peaks or transients at mid or high frequencies it soft clips less abruptly than an amp with lots of NFB .
Its not so much an effect Im looking for ,more a graceful overload when the convertor is approaching 0DBFS ,
which under normal circumstances in recording your nowhere near , intentionally anyway .

Theres also the simplicity and the fact that you can swing four times the supply voltage across the output devices in push pull .
As we said complimentary pairs are fundamentally different devices , and usually a large amount of overall negative feedback is needed to iron things out , when an amp like that starts to overload the NFB can longer work as intended , everything goes hellter skelter .

Another downside of the class B push pull arrangement is the transistors are likely to need matching to some degree , or performance could suffer .
Perhaps its less of an issue with class a bias , as the transistors are always on .