Need a 1 meg input pre stage-?!

colwaiting · 10th March 2011, 06:39 PM

I am new to posting here ('cept for selling stuff) but am ready to take the plunge!
been lurking for ages.

I have scoped out the OPA 2134 for my first stage. total gain (over several stages) must be ? 80? There seems to be quite a range of problems when working at 1 meg input. (I got, allegedly, 2.0 mV @ 1 megohm to amplify)

The OPA spec sheet says to correlate your input imp with the resistance in your feedback loop, but that might mean 1.5 meg//250K (to ground) which seems a bit out of whack.
Also it would appear difficult to get much gain in this stage, also. that i could live with, but 9 or 10 sure would be nice.

Do i need a Jfet or complementary Jfet input buffer here?
I have no design skills to speak of --just cookbook competence.

Advice, please. I'll happily use a 6DJ8 if I have to. Would i know what Im doing? No.

I am also correct that the inverting input is out of the question?

juma · 09:08 PM

You can use SE JFET buffer (see attachment). Input impedance is 1M and you can load this buffer with 5k or more. P1 (multiturn pot) sets DC offset.
Mouser sells BF862 JFETs.

colwaiting · 11th March 2011, 12:36 AM

that is very useful AND I see that there are no boutiqe parts!
It would be good to expand my knowledge of fet's

I have an excellent text (Horowitz & Hill) but nothing clicks
until I put it into wire and bits.

Doug

AndrewT · 11th March 2011, 09:47 AM

Hi,
a gain of 80 times (+39dB) for an opamp is not too much to ask.
A FET input opamp can easily achieve 1M for Zin.

The opa2134 meets both requirements and it is unity gain stable.

colwaiting · 15th March 2011, 12:24 AM

Oh good I was waiting to hear that.

I will try setting that up. I would have to guess why Burr Brown warns against excess noise & distortion. Would you have a suggestion for R1//R2? something like 500K/ 62K?

AndrewT · 15th March 2011, 12:28 AM

Quote:

a suggestion for R1

Is that a Yamaha?

colwaiting · 15th March 2011, 03:25 AM

the 2 mV 1 meg I connect to comes from the MPX board of a McIntosh 113 tuner. Also MR73

colwaiting · 12th April 2011, 01:44 PM

Long delayed thanks to Juma! I see that Q8 is set up as a current source??
Done some more reading in Horowitz et al. I had previously no real notion of Jfets.

Am I to assume that distortion is not an issue? Normally i think of any amplifier stage as being enclosed in a feedback loop--

My text sez "you can build discrete Jfet stages, but normally one looks at Op-amps that use Jfets in the input stage." Hello TL072?

juma · 13th April 2011, 12:27 PM

Quote:

Originally Posted by colwaiting

... Q8 is set up as a current source??

Yes, Q8 represents a working load for Source Follower Q7.

Quote:

Originally Posted by colwaiting

... distortion is not an issue?

Not at all.

10th March 2011, 06:39 PM	#1
colwaiting diyAudio Member Join Date: Dec 2002 Location: boston Ma.	Need a 1 meg input pre stage-?! I am new to posting here ('cept for selling stuff) but am ready to take the plunge! been lurking for ages. I have scoped out the OPA 2134 for my first stage. total gain (over several stages) must be ? 80? There seems to be quite a range of problems when working at 1 meg input. (I got, allegedly, 2.0 mV @ 1 megohm to amplify) The OPA spec sheet says to correlate your input imp with the resistance in your feedback loop, but that might mean 1.5 meg//250K (to ground) which seems a bit out of whack. Also it would appear difficult to get much gain in this stage, also. that i could live with, but 9 or 10 sure would be nice. Do i need a Jfet or complementary Jfet input buffer here? I have no design skills to speak of --just cookbook competence. Advice, please. I'll happily use a 6DJ8 if I have to. Would i know what Im doing? No. I am also correct that the inverting input is out of the question?

15th March 2011, 12:24 AM	#5
colwaiting diyAudio Member Join Date: Dec 2002 Location: boston Ma.	Low level input--Low noise-- OPA 2134 Oh good I was waiting to hear that. I will try setting that up. I would have to guess why Burr Brown warns against excess noise & distortion. Would you have a suggestion for R1//R2? something like 500K/ 62K?

15th March 2011, 03:25 AM	#7
colwaiting diyAudio Member Join Date: Dec 2002 Location: boston Ma.	yamaha ? the 2 mV 1 meg I connect to comes from the MPX board of a McIntosh 113 tuner. Also MR73

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11th March 2011, 12:36 AM	#3
colwaiting diyAudio Member Join Date: Dec 2002 Location: boston Ma.	that is very useful AND I see that there are no boutiqe parts! It would be good to expand my knowledge of fet's I have an excellent text (Horowitz & Hill) but nothing clicks until I put it into wire and bits. Doug

11th March 2011, 09:47 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, a gain of 80 times (+39dB) for an opamp is not too much to ask. A FET input opamp can easily achieve 1M for Zin. The opa2134 meets both requirements and it is unity gain stable.

12th April 2011, 01:44 PM	#8
colwaiting diyAudio Member Join Date: Dec 2002 Location: boston Ma.	Long delayed thanks to Juma! I see that Q8 is set up as a current source?? Done some more reading in Horowitz et al. I had previously no real notion of Jfets. Am I to assume that distortion is not an issue? Normally i think of any amplifier stage as being enclosed in a feedback loop-- My text sez "you can build discrete Jfet stages, but normally one looks at Op-amps that use Jfets in the input stage." Hello TL072?

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