I'm Back- sort of

[Audioguru]

With a gain as high as 201 times and without diodes to limit the output swing to 1.2V peak-to-peak, the opamp output will be 7V peak-to-peak (when the opamp supply is 9V) that might overload your amplifier input, because the opamp output will be clipping like crazy making squarewaves. An opamp output that is clipping sounds harsh, but with diodes in the feedback loop the clipping is still distortion but sounds softer.

All old vacuum tube guitar amplifiers had an input resistance of at least 1 million ohms to produce high frequency sounds of the pickup.
A lower load in the pickup reduces high frequencies causing a muffled sound. The old 741 and 4558 opamps had a minimum input of 300k which muffles the sounds a little.

The distortion pedal circuit in post #107 has an extremely low input resistance that needs a transistor buffer to increase it.

 

[SparkyCal]

Thanks AG. So, if I plan to do the 201 gain schematic, until I install the diodes, would it help to insert a volume pot somewhere to tame the output, until such time that I am in the diodes phase? If so, where would the pot go?

Thanks

 

[SparkyCal]

Also, the schematic calls for a 200K resistor. I don't have oen. I have the follwojgmn:

2 100Ks that can be installed in series

a 220K

a. 180K

Which do you think would be best?

 

[Audioguru]

Doesn't your power amplifier have an input volume control?
A guitar or a pedal has a volume control that is near you to be adjusted while you are playing the guitar.

 

[bertus]

Hello,

In post #121 I showed how you cn calculate the gain.
Any combination will work.
The gains will be 201 , 221 or 181 times.

Bertus

 

[Audioguru]

200k feeding the 1k resistor to ground produces a gain of 201 times.
180k makes 181 times and 220k makes 221 times. The 10% difference will not be noticed. Try the 220k.

 

[SparkyCal]

Thanks Bertus and AG. Bertus- yes, you did. That formula seems very clear and straightforward.

Yes, my amp and may guitar both have onboard volume,e controls. I will use them until I build, this out further to include pots.

 

[SparkyCal]

btw..I just received these. I forgot I ordered them as it was over a month ago. They just arrived:

https://www.amazon.ca/gp/product/B08XPQHCZ2/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1

https://www.amazon.ca/gp/product/B012ACSC1O/ref=ppx_yo_dt_b_asin_image_o06_s00?ie=UTF8&psc=1

 

[Audioguru]

Since Amazon and their seller know nothing about electronic parts and the manufacturers of the diodes are no-name-brand then some of those diodes might not work and should be tested.

 

[Martaine2005]

I'm not sure why people use Amazon for electronic components. I am told because of the fast delivery. Not in your case.
The sellers seem to buy from Ebay and resell. IMHO it's much cheaper from ebay.
This is just one example of 1N914. for 500pcs.
And another for 1N4148. Only this lot is for a 1000pcs.

Martin

 

[SparkyCal]

AG. I did not look at the first link, but the second one is from china. https://www.ebay.co.uk/itm/221890304544?hash=item33a9b15a20:g:OvoAAOSwuTxV~SR~#shpCntId

I hear you about products from China. But I'm sure we could assume they are all bad. I have 16 guitars. My American made ones are all quality built. The Korea and China ones vary. But most have a reasonable quality for the price point. I only bought one bad one, that was made in China. The sound hole ring actually fell off. It's also not that easy finding stuff in. Canada. For example. I tried finding the TL081 op amp on digikeys and could not. But I digress.

I build the new 201 circuit and am not getting any sound. Here are the pin readings:

1. 8.35
2. 38.4
3. 4.10
4.0
5.4.10
6. 4.5
7. 4.5
8.9.03

Houston, we have a problem. Pin 2 is at 38.4. that seems very high.

 

[bertus]

Hello,

I assume you are using the 4558.

The voltage on pin 2 can not be.

Bertus

 

[Audioguru]

On another website, somebody is buying 1000 cheap diodes from Amazon. Somebody else said that the Amazon seller is buying cheaper diodes (more defectives and fakes) from ebay and re-selling them on Amazon.

Today, digikey.ca in Canada has 9100 TL081CP opamps from Texas Instruments for $0.86 each. The top of the page says, "Free shipping on orders over ...." but is cutoff.

I think your circuit with a gain of 201 (or is it 221?) has the capacitor missing again and you measured 38.4mV.

 

[SparkyCal]

I will have a look and report back guys. I will also check out digikeys again. I don’t know why my search did not return a result on the tl081. Yes Bertus, I am using the 4558d. I should have said that.

 

[SparkyCal]

I had a quick look.

The Out of the first op amp (which in Pin 1), goes first to the 220K resistor which then travels to Pin 2 (the feedback loop)

The Out then attaches to the 1K resistor and then to the 1Uf capacitor, that goes to ground.

 

[Audioguru]

The opamp output pin1 does not connect to a 1k resistor.
I think the capacitor to ground is missing again:

 

[SparkyCal]

Leave it with me. I need to take a closer look when I get a chance. Will report back. Thanks!

 

[SparkyCal]

I was excited as I was about to order 20 Texas instruments op amps from DigiKey. The prices were very good. But when I got to the payment page, it wanted to add an additional $20.00 in fees for getting it here ;-(. It's too bad. It would have been a great choice.

 

[SparkyCal]

We are making progress.

You were right. I forgot to add that same capacitor (I'm not sure why I keep missing that).

Anyway, after installing it, it worked. I am getting distorted sound out of the op amp. The pin readings are below. I used a new 4558D, just in case I blew the last one.

The only issue is volume. For example. If I plug my guitar directly into the desktop Orange guitar amp, I get plenty of volume. When I turn up the distortion dial on the orange amp, I get plenty of distortion and volume. It's loud and dirty.

With the distortion dial all the way down, and with the volume all the way up on the Orange amp, and going through the op amp circuit we are building. I hear distortion, but the volume is nowhere as loud as it should be. It's dirty but not loud.

So, the goods news is, it's working. The bad news is, not enough volume to drive the amp. Before proceeding any further, I think this needs to be figured out, otherwise, I don't think the circuit will be useable.

1. 4.56
2. 4.58
3. 4.15
4.0
5.4.15
6. 4.57
7. 4.57
8.9.03

 

[SparkyCal]

Since you folks have been kind enough to coach me during my struggles, I thought, at the very least, I should let you hear what it sounds like.

Although these little desktop amps are small, they pack quite a punch. Have a look here and listen at 3:17

The following clip is with my desktop guitar amp at full blast with no distortion coming from the guitar amp. The distortion is from the op amp circuit. You will notice there is quite a lot of buzz (because I have the guitar amp at full blast, and the sound is very low in volume compared to the Youtube clip. At a proper volume, it should pump out the same punch as in the video. By the way, I used the TL081 (which has the same pin settings as the 4558D. I also used the 4558D, with no appreciable difference. The clip is using the TL081 op amp, btw,.

https://drive.google.com/file/d/1m83as_h9-Pq8zcF1AAbSVDumBv7L-i56/view?usp=sharing