Resistor and Pot in parallel not working as I expected. - Page 1
Forum section: electronics / projects

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 03:43:03 am
content: I am working on a guitar headphone amp using an LM386. I am basing my design off of this onehttps://www.electrosmash.com/1wamp. But I didn't have any JFETs lying around so I built the preamp from this sitehttp://www.rason.org/Projects/bipolamp/bipolamp.htm. It's all working well except the adjustable gain. I set it up like in the 1Wamp schematic using a 50k linear pot with a 1k resistor in parallel between pin 1 and 8. However the problem that I am getting is that it is acting more like a 2 level gain. With the pot turned all the way to one end it is at a low gain setting (say 80) then when I get about 1/5 from the other end it is all of sudden high gain (200). I also bought a 50k audio pot just in case that was the problem but it also had the same issue. I also tried a 100k pot and a 10k pot and they all had the same issue. I also tried the audio pot and liner pot by themselves with no luck. I also hooked the parallel resistor setup, out of circuit, to a meter and the resistance varied correctly.I would like to have an adjustable gain with a somewhat smooth curve. Any ideas on what I am doing wrong?I have attached a schematic of my circuit.

author: hamster_nz
author_rank: Super Contributor
timestamp: January 06, 2018, 04:30:49 am
content: Did you use a linear pot? A log pot is more common for audio gain control

author: Tom45
author_rank: Frequent Contributor
timestamp: January 06, 2018, 06:47:16 am
content: Plot the curve for parallel resistors R1 = 1K, R2 = 0 to 50K using linear steps (say 500 ohm intervals for the pot).You will see that for most of the pot's range, the effective resistance is nearly 1K. Only when the pot gets down to a few thousand ohms will it have an appreciable affect on the effective resistance.Why do you want to use the resistor, pot combination?

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 12:49:08 pm
content: Quote from: hamster_nz on January 06, 2018, 04:30:49 amI tried both.

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 12:50:37 pm
content: Quote from: Tom45 on January 06, 2018, 06:47:16 amWhen I saw the design I thought it made sense. But now that I have graphed it like you suggested I see that is not the case.Thanks.

author: Lee Leduc
author_rank: Regular Contributor
timestamp: January 06, 2018, 01:13:25 pm
content: Here's a couple of articles about modifying pot tapers. Google "changing pot tapers" to find more info.http://electro-music.com/forum/phpbb-files/pot_taper_amz_168.pdfhttp://www.geofex.com/Article_Folders/potsecrets/potscret.htm

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 04:01:25 pm
content: Quote from: Lee Leduc on January 06, 2018, 01:13:25 pmThanks for those. They look very interesting.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 06, 2018, 04:55:51 pm
content: Note that the GAIN nodes (pin 1 & 8 ) are NOT very useful for adjusting USER VOLUME. They are intended for setting the gain of the LM386 stage which is typically a static, circuit-design value.Strongly suggest using a conventional volume control as in your R11. Although your circuit seems a bit convoluted compared to the common implementation.Note also that connecting pins 2 and 3 together on J2 (the guitar input) will likely short out the audio signal (on the tip, pin 2) to ground (the sleeve on pin 3).

author: Audioguru
author_rank: Super Contributor
timestamp: January 06, 2018, 05:12:22 pm
content: Did you notice that the Jfet guitar preamp and almost every vacuum tube guitar preamp has an input impedance of 1M ohms or more so that the high impedance pickup is not muffled?Did you also see that the microphone preamp circuit you are using has a fairly low input impedance?I agree that a gain control should be fixed and a volume control should be used to adjust levels.

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2018, 05:56:53 pm
content: The polarity of C1 is incorrect. It's reverse biased, so will leak bias current from the input potential divider to whatever is the source is.What's going on with R9 to R11 and C3 & C4? It looks like a tone control, but beware it will load down the previous stage considerably.R17 in series with the headphones is a bad idea, because it will result in poor damping factor. Ideally the output impedance of the amplifier should be as low as possible. If the output level is too high, then reduce the gain. You'll be better off reducing the gain by a factor of 22 and get rid of R17.C8 is too large for a zobel network. Make it 10nF to 100nF.What over all gain do you require? It's quite likely one or both of those common emitter amplifiers aren't needed.What input impedance do you require? The BJT amplifier will have a much lower input impedance, than a J-FET amplifier.

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 10:07:01 pm
content: Quote from: Richard Crowley on January 06, 2018, 04:55:51 pmPin 2 and 3 are not connected. There is no dot connecting them. (I used EasyEDA and that is the convention on that one).R11 is tone control not volume.

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 10:09:14 pm
content: Quote from: Audioguru on January 06, 2018, 05:12:22 pmNo I didn't, I am just a beginnerThanks for the info I'll look into that.I did not know that gain control should be fixed. The 1W amp example I used had a variable gain so I thought it would be ok.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 06, 2018, 10:15:52 pm
content: Quote from: markbernard on January 06, 2018, 10:07:01 pmYou appear to be mistaken. I clearly see a connecting dot between J2 pin 2 and pin 3. Just to the left of the (backwards) C1.Your schematic diagram shows a TRS connector for J2 while the title says "6.35mm mono". Perhaps that is the source of your confusion.QuoteIt would be a very good thing to label it properly.

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 10:21:15 pm
content: Quote from: Hero999 on January 06, 2018, 05:56:53 pmThanks. I didn't even notice. The original 1W amp schematic has them in that configuration but it also feeds into pin 2 not 3. Switched to pin 3 because it sounded better. But I may have had to adjust the gain instead.Quote from: Hero999 on January 06, 2018, 05:56:53 pmIt is supposed to be a tone control. Do I need to use a buffer to fix it?Quote from: Hero999 on January 06, 2018, 05:56:53 pmWill do.Quote from: Hero999 on January 06, 2018, 05:56:53 pmThat is a typo. I for got to change the default value. It is 100nf.Quote from: Hero999 on January 06, 2018, 05:56:53 pmI just want enough output to hear my guitar mixed with the extra audio input. I'll pull r17 like you suggested and it may helpsound fine with the base gain of 20.Quote from: Hero999 on January 06, 2018, 05:56:53 pmI am not sure. It is my electric guitar. I don't know if they are high or low. I would have to search.I went with the NPN transistors for the preamp because my local electronics store didn't have any JFETs and I didn't want to wait 2-4 months for EBay.Is there a way I can still use these transistors?Thanks for all the great information. I'll make the suggested changes and read some more.

author: markbernard
author_rank: Contributor
timestamp: January 06, 2018, 10:23:26 pm
content: Quote from: Richard Crowley on January 06, 2018, 10:15:52 pmSorry my mistake I thought you were talking about the chip. I didn't want to spend hours looking for a mono input so i just took the stereo one and connected the 2 pins together. (At least I thought it was a stereo input.)Quote from: Richard Crowley on January 06, 2018, 10:15:52 pmYes thanks. I will add a label.

author: markbernard
author_rank: Contributor
timestamp: January 07, 2018, 02:33:35 pm
content: Now I feel foolish. I remember I do have MPF102 and tried them but lost patience and moved on to S9014. I am giving the MPF102s another try but they produce a lot of noise (compared to using the LM386 without a preamp or with the one I made above).I tried a few that I found so far but I will keep trying some more. If anyone has any designs that worked for them please let me know.Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2018, 09:08:31 am
content: This isn't something I've tried to do before, but I've participated in similar threads, so have a fairly decent understanding of what's required.The guitar probably doesn't need much amplification, before the LM386. It may even need some attenuation, to avoid overloading the LM386 and driving it into clipping.You may need to review the tone control. With R11's wiper at the top, it's a high-pass filter with a cut-off of around 160Hz and when it's at the bottom, it's a low-pass filter with a cut-off of 160Hz. I don't know whether this is what you want, but I've not seen this before.What's the auxiliary source from?How about replacing the transistors with an op-amp? Do you have the TL072?

author: markbernard
author_rank: Contributor
timestamp: January 08, 2018, 12:05:57 pm
content: Quote from: Hero999 on January 08, 2018, 09:08:31 amIt will be a line input. In my case I want to feed from my computer. The line input is not giving me any problems.I don't have an opamp. I built a portable amp with just the LM386 before. It worked ok but the tone is off a bit and it's not stable at higher input volume. I wanted to add the preamp to stabilize it. It worked with the NPN transisters. I could set the gain higher and still not get distortion. But it was drawing around 50ma, which isn't bad.Trying the preamp with the MPF102 dropped the current use considerably and the tone improved over the NPN preamp. But now I am getting a lot of noise. I've searched and found a bunch of different ways to do it with JFETs. No matter which one I try I still get the noise(kind of like static). The noise doesn't change whether I am playing or not, it is just a constant white noise.

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2018, 11:24:22 pm
content: You can't just swap the BJT for a J-FET and expect it to work the same. Please post your updated schematic.The LM386 should give a stable gain. The datasheet shows how to calculate the gain.http://www.ti.com/lit/ds/symlink/lm386.pdf

author: markbernard
author_rank: Contributor
timestamp: January 14, 2018, 11:36:45 pm
content: Here is the JFET version I tried. I played a bit with the values but this is essentially it.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 15, 2018, 12:16:48 am
content: The input connector, J2 has pin 2 and pin 3 shorted together. That will short the input signal to ground with a typical TS guitar cable.You are not going to include a volume control? A 50K audio-taper pot would be appropriate between C2 and U1.

author: markbernard
author_rank: Contributor
timestamp: January 15, 2018, 12:48:39 am
content: They are not really shorted. I was just trying to show that it was a mono input.I didn't try a volume control yet. I was just trying to get rid of the white noise. But I went back and tried the NPN version and it also had white noise so I may have blown something somewhere. So I'll have to check.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 15, 2018, 01:44:57 am
content: Quote from: markbernard on January 15, 2018, 12:48:39 amIf you wire the connector like that it is shorted and you will get no signal.Your diagram does not show "a mono input".QuoteAll circuits have white noise. Especially high impedance circuits. Alas, that is one of the unfortunate legacies of 60-year old electric instrument technology. The microphone industry moved on to low-impedance balanced. To bad the e-instrument industry is still stuck back in 1954 with high-impedance, unbalanced. The critical factor is the signal-to-noise RATIO. How far down is the noise compared to the the expected signal level?

author: markbernard
author_rank: Contributor
timestamp: January 15, 2018, 11:53:40 am
content: Quote from: Richard Crowley on January 15, 2018, 01:44:57 amWhen I am playing you can't hear the white noise. When I was doing the first circuit there is no white noise, just a barely audible hum. The hum I got using NPN is the same hum you hear when you plug a guitar into a regular store bought amp. The white noise I got with the JFETs is like when you turn your TV on looking for an analog channel and there is just snow and white noise.I am trying the NPN again with higher input impedance and it is working well. I will post that soon if anyone wants to weigh in.Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2018, 02:46:16 pm
content: It's possible the BJT, being a lower impedance, is cutting the high frequencies, which is why there's less hiss. It doesn't mean the sound quality is any better.EDIT:To expand up on that. The impedance of a guitar pick-up is inductive (between 1H and 10H, according to the articles linked below) so will be proportional to the frequency. If it's loaded, with too lower resistive impedance, then it will cause higher frequencies to be cut.http://doylecoils.com/Pickup%20Primer%20Pt%20IV.pdfhttp://www.guitar-repairs.co.uk/how_guitar_pickups_work.htmFC= R/(2pi*L)So if your amplifier has an input impedance of 2k and the amplifier an input impedance of 2000 Ohms:FC= 2000/(2*3.142*2) = 2000/(6.284*2) = 20000/12.568 = 159Hz.Higher frequencies will still be audible. They'll just be attenuated by half, every time the input frequency doubles FC. It might sound a bit muffled.Have you tried a bootstrapped BJT emitter follower? The attached schematic has an input impedance of about 1M, depending on the transistor's Hfe. I can't see it being less noisy than the J-FET amplifier. Test it.

author: markbernard
author_rank: Contributor
timestamp: January 15, 2018, 06:39:33 pm
content: Quote from: Hero999 on January 15, 2018, 02:46:16 pmThanks for that. I'll give it a try.I did notice that the high tones sounded muted. That is why I was adding the tone control. So I definitely have to bring the input impedance up.Thanks.