Simplistic NJFET RIAA

[RCruz]

Here it is Rev 1 with a reviewed gm factor.

 

[Salas]

Thank you

It was really mented as a tool for experimentation and comprehension... as is it is no substitute for a simulator and I really believe we should always breadboard before building anything.

Anyway, The scale factor of 2 in gain Equations is nominal. According to the National Semiconductor FET Handbook (1977), that factor can range from about 1.1 to 2.5 but is typically near 2, so I can easily adjust it based on your input.

Can you give me some gain figures ?
48dB build 28V output stage Rd 4k7 Vd = 11v and Rs 47r
48dB 28V input stage Rd 3k3 Vd = 14v and Rs 33r
57dB 37V output stage Rd 6k3 Vd = 10v and Rs 27r

I believe these will beenough to find a good compromise for the gain equation.

I only have the folded on breadboard right now. You should measure your builds too to see about deviations. A test record and a DMM on AC are enough. Probe the 2nd stage's input and output and you calculate gain by the out/in division.

 

[buzzforb]

Ok fellas, first channel fired up. Something not right. 35V rail. 8.1V at base of bc560. 5.9v at top of Jfet. 8.1through 3.9r sorce resistors of jfets. Nothing at collector of bc. I am leaning towards bad bc560.

 

[Salas]

Recheck the BC its a PNP and its correct orientation first. All the circuitry realization around first stage for that matter. You should get 8.1V plus Vbe on top of the input Jfet(s) and 25-30mV across each Rs.

 

[buzzforb]

I have 30mV across Rs, but only 5.8V at drains of jfets. Will double check orientation.

 

[buzzforb]

Gonna replace bc560, as all else checks out.

 

[Salas]

Maybe you overcooked something with the iron?

 

[buzzforb]

I am in the bermuda triangle here.

Top of BF245 12V
Base and emitter of BF245 - 8.1V
Used pot to get emitter of bc560 to 8.1V
Nothing on collector.
Jfets have 31mV acorss 3.9R source resistor.
Output gain fet has 4.7V across drain. Same thing at output, as the cap and pull down resistor are not added yet.
16V at top of the buffer.
R5/6 sitting at 6.71K
This thing should be working.

Gonna try the other channel. Just does not make sense.

 

[Salas]

Its all over the place.

 

[buzzforb]

Its all over the place.

Your saying mulitple things are wrong?

 

[Salas]

Yes. You should have half the BC560's emitter to ground voltage on its collector to ground after trimming, ~10V across second stage's Jfet drain to ground, for 35V to ground B+. Also the BF245 looks like drawing much. Either isn't an A type (lower IDSS rank) or its a short.

 

[buzzforb]

I have bf245b. I have 4ma of current on Jfet gain stage after 43R degeneration. Both channels exactly the same. In terms of general settings and measurement. Is it the bf245b?

 

[Salas]

Check its legs in its datasheet, its not like Toshiba pin order in case you did not, also take them down to 5mA with a resistor on their gate to source, feeding the leds from gate. See from there on by measuring again.

 

[buzzforb]

Are you saying insert a degeneration resistor into current layout. The led's are just a Vref for the bjt, being fed by bf845 current source, correct.

 

[Salas]

Correct. Or use a K117GR if there is no space for a resistor. Only be sure about orientation. If its wrong or type is changed, do the Irish dancer leads with a pliers.

 

[samoloko]

when you use obbligato premium gold as a coupling which end Is recommended to be In and which out for signal
do you recommend bypass of obbligato premium gold 4.7 uF - If you do which bypass and value - teflon ft-1 ft-2 ft-3

 

[RCruz]

when you use obbligato premium gold as a coupling which end Is recommended to be In and which out for signal
do you recommend bypass of obbligato premium gold 4.7 uF - If you do which bypass and value - teflon ft-1 ft-2 ft-3

I normally use the cap orientated as per it´s casing lettering.
input -------[Obbligato]-------- output

I would start without any bypass. Let the caps settle and see if you like the sound. I once bypassed one of those with a smaller obbligato (0.47u) with good results.

 

[RCruz]

Hi buzz

Here you have my measurements... appart from output buffer, it is the same as yours.

 

[samoloko]

thank you for reply
I have listened at least 100 Hrs but still think that Is missing air

 

[RCruz]

thank you for reply
I have listened at least 100 Hrs but still think that Is missing air

In that case a 100n FT-3 will open it wide.