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Old 13th April 2016, 11:24 PM   #1
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Default Do you think this JFET BJT pre-amp will work ok?

Hi all, thanks for looking.

I'm trying to build a preamp that has an input impedance of about 1Meg, and output impedance of about 1k or less. I know this can be done easily with opamps, but I'm new to op amps and still learning to use them well. (Thanks to all the thoughtful comments and suggestions about op-amps I have received)

Here is a pre-amp I came up with, it is just a jfet buffer with impedance of 1 Meg (and output impedance of 356ohms) fed into a BJT class A amplifier with gain range of 1-8 (see chart). It has an input impedance of roughly 10k and output impedance of 1k.



The reason I'm wanting a circuit like this is for several reasons. I have various amp units that have gains of 12-32 and input impedances ranging from 10k to 100k. I also have a distortion circuit that has an ouput impedance of 100k. So anyhow, this I believe would allow me link these higher impedance guitars, distortion unit etc, into the lower impedance amps. It would allow me to bump the gain up too.

I will eventually get better with op amps, but when I do get them working right, they sound more sterile and too perfect for me, but that's kind of subjective I suppose.
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Old 13th April 2016, 11:31 PM   #2
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Quote:
Originally Posted by garybdmd View Post
Hi all, thanks for looking.

I will eventually get better with op amps, but when I do get them working right, they sound more sterile and too perfect for me, but that's kind of subjective I suppose.
An op amp circuit can have as much colouring as you like.

I use a op amp soft limiter with my guitar and its harmonics vary from minimal to quite rich.


I have used a similar circuit to yours in a record player pre amp and it sounded very good.
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Old 14th April 2016, 12:15 AM   #3
GoatGuy is offline GoatGuy  United States
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[1] 9 volt operation is possible, but has quite a few limits, especially as the batteries inevitably run down. These days, I prefer using 2 little plastic battery-holders, each of which can take 6 batteries of the AA size. That way I can use “enerloop” type rechargeables that don't lose their charge over time. They might only be 1.3 volts, but 12 of them come in at ±9 VDC, with a +9, 0, –9 set of rails.

[2] I also like to use the first stage's JFET as the actual amplification stage (with inversion), and the BJT as the 0 dB emitter follower. Its working in a small enough range that it is quite efficient.

[3] And I like using the 2N5460 p-channel JFET “in parallel” with the 5457 to get amplification symmetry for those peaks both positive and negative.

If you are using this for a plucked instrument, I advise keeping the overall gain down. Pickups are remarkably hot these days. It is easy for a pickup to produce 5,000 millivolt peak-to-peak signal.

Anyway, that's about all.
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Old 14th April 2016, 12:42 AM   #4
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Default lowered values of voltage divider resistors

Thank you GoatGuy for the response. I'm kind of new to JFETS having been playing with them for a few days. I kept getting too high of output impedances on the jfet initial stage and I was slightly more experienced working with the bjt (as I've been fooling with them off and on for a month). But I think your idea is better once I get a little more practice with the JFETS.

I did notice something about my preamp, the current going down the voltage divider resistors is not greater than 10 x's that going across the base when I do the simulation, so I lowered these resistors and am attaching that jpeg as a correction. I think that's a general rule in design.
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Old 14th April 2016, 09:17 AM   #5
AndrewT is offline AndrewT  Scotland
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I don't think you need the biasing resistors at the input/gate of the jFET Buffer.
Does it self bias?

Have a look at Juma's BF820 amplifier.
It is doing what Goatguy is suggesting as an alternative.
And aim for a maximum output of roughly half the battery supply voltage.
i.e. if you assume 5000mVpp as input and apply a maximum gain of +10dB (3times) you have a maximum output of 15Vpp. Use a battery that gets close to +-15Vdc
If you only have ±9Vdc, then think about maximum output levels of 10Vpp (3.5Vac, +13dBu)
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Last edited by AndrewT; 14th April 2016 at 09:19 AM.
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Old 15th April 2016, 02:11 AM   #6
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Default eliminated 1 bias resistor on jfet stage

Thanks, I will look up Jumas amplifier. I did remove one biasing resistor and attached the results. It raises the output impedance of the jfet stage a bit, so I had to step up the bias resistors a little at the bjt stage, which leaves the base to bias current ratio to be 9 and I think they say, 10 is minimum typical, but it probably won't matter. It lowers the gain a tad also. I will try the prototype in a week or so, back to work for me.
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Old 15th April 2016, 06:16 AM   #7
xrk971 is online now xrk971  United States
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Default It works

Click the image to open in full size.

I just built the above circuit on perfboard (with 85.7ohm Re), but used a 2SK170 JFET as that was all I had. I added a 7812 regulator for the supply and used 10uF electrolytics bypassed with 1uF film caps. The input cap is 0.1uF film. I don't have an O-scope but tested it by using some 60ohm headphones (I know, impedance was way too low) and got sound to come out at low volume. What I heard was clear.

Click the image to open in full size.

Edit: I tried using it as a pre-amp between my CD player and my Circlophone amp. Sounds very nice - very transparent class A sound. Works well.

This pre-amp confirms that my bag of cheap qnty 40 x 2SK170's from Aliexpress actually work.

This may be just the ticket as a pre-amp for my Pass ACA amp. Same 2SK170 front end, same SE class A operation, simplicity with few parts.
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Last edited by xrk971; 15th April 2016 at 06:43 AM.
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Old 15th April 2016, 09:02 AM   #8
AndrewT is offline AndrewT  Scotland
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Measure the Idss of the jFETs.
Then measure the Vp.
You can use those to determine gm. Borbely shows how to measure and calculate.
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Old 15th April 2016, 10:36 AM   #9
xrk971 is online now xrk971  United States
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Quote:
Originally Posted by AndrewT View Post
Measure the Idss of the jFETs.
Then measure the Vp.
You can use those to determine gm. Borbely shows how to measure and calculate.
Do you mean measure the JFETs bare outside of circuit to determine if any good?

Reading up on article by Borbely - looks like changing the input single JFET to follower with constant current source using same JFET allows zero DC offset, so we can skip the AC coupling cap plus reduce HD as shown in Fig 15C in article below.

http://www.linearsystems.com/assets/...%20Bordely.pdf

Fascinating article as it shows that with two SK170's as connected in Fig 16A, you can drive low impedance loads (2.3ohm output impedance) and completely skip the output BJT. If matched devices the THD is very low and will be pure class A as long as output current is less than half quiescent current.

Last edited by xrk971; 15th April 2016 at 11:02 AM.
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Old 15th April 2016, 11:26 AM   #10
AndrewT is offline AndrewT  Scotland
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Yes, check your ali jFETs. I can't believe that Ali sells real 2sk170 for cheap prices.
The DC coupled requires a dual polarity supply.
Also read D.Feucht and Pass B1 and DCB1
But I still think you should do what Goatguy suggested.
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File Type: pdf Feucht Buffer.pdf (41.7 KB, 63 views)
File Type: pdf DFeuchtWhyOscillate.pdf (120.9 KB, 36 views)
File Type: pdf FeuchtCascodeAmplifiers.pdf (166.4 KB, 48 views)
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