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Oct 24, 2021 - 12:43:29 PM

FiFi

UK

18 posts since 5/25/2015

Hi, just wondering if anyone can offer advice on getting a good amplified sound?

When gigging/rehearsing, I'm using a Schertler Jam 100 amp (https://www.thomann.de/gb/schertler_jam_100_anthracite.htm) and my fiddle is fitted with a 'Skyinbow' pickup (http://www.scayles.co.uk/ProductDetails?ID=NDU4NA==). The amp was highly recommended to me by a fiddle playing friend and same with the pickup, I had numerous recommendations for this particular one. The problem I have is, once my amp is turned up to roughly 40% volume, the sound is distorted, with excessive vibration/vibrato. This only happens when I play 'double-stops', when I play a single string there's no distortion, it sounds fine. As you can see in the link, the only control settings on the amp are high, mid, low, fx, and volume, I have the high, mid and low set down low and fx barely on. A friend suggested I try linking a compressor to the set-up but that didn't help.

Thanks in advance for any help but can you please keep it simple as I'm not a tech junkie! smiley

Fiona

Oct 25, 2021 - 6:42:59 AM
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2393 posts since 10/1/2008

OK .... there are likely problems with either your instrument, or your using of the added equipment. Has your friend that has similar equipment checked out your "usage" in comparison to his own? You may have a loose joint or spot on your bass bar that is unheard but picked up by the equipment. It is possible that you may need a preamp that you can use to 'disregard" given frequencies if there are just one or two spots where this happens. Otherwise it is time to have a qualified luthier inspect your instrument. Lastly IDK check all your cords and connectors. Electricity , at times, seems to have "magical" properties. Good luck. R/

Oct 25, 2021 - 8:49:34 AM
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LukeF

USA

102 posts since 10/15/2019

The installation of this pickup is a bit tricky and that is why Skyinbow recommends having a professional luthier install it. Did you install it yourself? If installed by the shop you bought it from, perhaps you should take it back to the shop to check if it was done correctly, or take it to a luthier.

Oct 25, 2021 - 10:42:46 AM
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RB-1

Netherlands

41 posts since 9/28/2020

I do suspect amplifier set up errors.

To find out, some questions first:

Which input did you connect  your pick up's output to?

How did you set the '40%' volume?

And why didn't you set the three EQ bands for a flat response first? Reason?

From there we can look into more specific advice.

Oct 25, 2021 - 12:31:02 PM

LukeF

USA

102 posts since 10/15/2019

Good point about checking for amplifier set up errors. I looked up the specs for this amp. I see that it has both a "Volume" and "Master Volume" control knobs. The volume knob could actually be a "Gain" knob. If you set the gain knob low, it would minimize distortion, no matter how high the master volume knob is set. Conversely if the gain knob is too high, that would invite distortion. So keep the volume knob low and use the master volume knob to adjust your volume output.

Oct 25, 2021 - 2:02:45 PM

2468 posts since 10/22/2007

The Skinbow pickup looks very similar to the Fishman bridge pickup. The need for professional installation is because the bridge needs fitted like any other bridge. If in the process, the bridge falls and snaps the cable at the bridge, your expensive bridge pickup is toast. Dunno the OP's installation, but it does seem more to lean towards the amp as the culprit. The input seems to be overdrove. Maybe there is a mislabeled gain pot? I also have various amp modeling choices/selector. From Acoustic, to Metal Stack. I also wonder if there is an effects percent pot. What effect(s)? Are they external or built in?

Oct 25, 2021 - 2:26:14 PM

DougD

USA

10336 posts since 12/2/2007

No, if I'm looking at the right pickup, the element fits into a 1/8" hole you drill in an existing (or new, in which case it would need to be fitted) bridge.
Sorry I don't know more about either of these pieces, but it sounds like you have either a level (volume) or impedance mismatch between the pickup and amp.
If this is the pickup with the preamp built into the chinrest, is it charging properly?
Maybe you can get together with your friend and try some mixing and matching to find the problem
Good luck!

Oct 25, 2021 - 4:16:23 PM

DougD

USA

10336 posts since 12/2/2007

I've never seen one of these amps, but I did find this review: acousticguitar.com/schertler-j...0-review/
It looks like there are quite a few input options - maybe it will help you sort it out. Also, it looks like that pickup system is pretty "hot" and it might be easy to overload an input. Then too, that's not really a very big amp, and it might not be capable of putting out the volume you want.
By all means set your EQ controls to the middle - that's where the starting point usually is.

Oct 25, 2021 - 7:54:20 PM

269 posts since 6/3/2016

The distortion on double stops is a classic overdrive sound. So either the amp is being overdriven, or something else earlier in the chain like your preamp is being overdriven. I'm looking at the manual for that amp.  https://www.schertler.com/public/download/user_guide_jam_en.pdf You are presumably using the Channel 1 instrument input, which is identified as 1.1 Instrument In in the manual. There is an Overload light marked "OL", marked 1.5 in the manual. When you play double stops or dig in hard with your bow on a single string, that light should not go on. If it does go on, especially when you are hearing the breakup, then you should turn down the gain, knob 1.4 in the diagram.

If you are going into Channel 2, it looks to have similar indicators and controls.

If you are hearing breakup, but the OL light is not going on, then some signal earlier in the chain is too hot. This could be the case if you had an external preamp. If so, then you need to turn down the volume / gain on the preamp or other widgets.

In summary:

  • Bow hard double stops and look at the OL light
  • If the OL light flashes on the amp, turn down the amp gain
  • If you hear overdrive breakup but the light does not go on, the input signal is saturating for some other reason

That looks like an awesome amp. I'm sure you can get it to work.

Edit: And focus on the first stage gain, not the volume. I would expect the overdrive to be independent of the volume setting. So doing this, I would first turn the volume down low. I would still expect to hear the overdrive. If it goes away, then the problem could be in an amplifier stage controlled by the volume knob. But that seems less likely.

Edited by - RinconMtnErnie on 10/25/2021 20:00:25

Oct 25, 2021 - 8:17:08 PM

269 posts since 6/3/2016

I looked up the output impedance of your pickup and the input impedance of your amp. The output impedance of your pickup is 1 Kohm. The input impedance of the instrument input of your amp is 10 kohm. In general you want the input impedance of the amp to be much higher than the output impedance of your pickup.

In theory you should be okay there without a preamp, so I'm guess that's what you're doing. Once again that would point to needing to adjust the gain.

Edited by - RinconMtnErnie on 10/25/2021 20:17:27

Oct 26, 2021 - 4:54:25 AM

RB-1

Netherlands

41 posts since 9/28/2020

I do agree with many of the suggestions above.

In order to take the guesswork out of the equation, I asked Fiona to come up with some data, from which we could easily deduct which way to go and thereby solving the issue.

Oct 26, 2021 - 7:33:54 AM
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DougD

USA

10336 posts since 12/2/2007

Ernie, have you looked at this pickup? Its unusual, but its an integrated system with a preamp built into the chinrest. It says the maximum output is 4v p-p though, which is a lot hotter than an average magnetic guitar pickup.
Fiona, as others have suggested, make sure the pickup is securely mounted in the bridge, and the bridge to the instrument. Also, these items, and even these brands, are uncommon over here, so I don't know how much help you'll find on this forum. If possible, I really think you should get together with your friend so you can plug your fiddle into their amp and vice versa.

Edited by - DougD on 10/26/2021 07:43:59

Oct 26, 2021 - 2:16:15 PM

FiFi

UK

18 posts since 5/25/2015

Thanks so much to everyone who's commented on here, I really appreciate it. I'll try to cover all the suggestions that have been made:

I've had a luthier do quite a bit of work on my fiddle in the past, I think if there was something up with the bass bar or something else, he would have noticed then.

I did have the same luthier install the pickup, he's been around for years/good reputation etc so I don't think there will be an issue with the fitting of it. The pickup has a preamp built into the chinrest, I re-charge it regularly with a battery.

I have tried different cables but no success there.

I have tried having the 'gain' knob way down low but that doesn't seem to improve the sound any.

I've tried connecting to both channels (1 & 2) with the EQ bands at all different settings but it doesn't improve the distortion.

I think, as some of you have suggested, I need to get together with someone else and maybe try a different amp too. I'm going to take all my gear to my friendly luthier's shop, see if he can help me get to the bottom of this. If we manage to solve the problem I'll let you know.

Thanks to everyone again!

Oct 26, 2021 - 4:26:12 PM

DougD

USA

10336 posts since 12/2/2007

Have you tried the inputs on channel 3? If you read the review at the link I provided above you'll see that finding the right input can be a little tricky. The reviewer found that a high output preamp worked best in the "Lo" input on channel 3. Might work for you too.

Edited by - DougD on 10/26/2021 16:28:40

Oct 26, 2021 - 8:06:49 PM
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5505 posts since 9/26/2008
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Two strings may also increase the output, wouldn't it?

Oct 27, 2021 - 9:01:06 AM

gapbob

USA

787 posts since 4/20/2008

quote:
Originally posted by RinconMtnErnie

I looked up the output impedance of your pickup and the input impedance of your amp. The output impedance of your pickup is 1 Kohm. The input impedance of the instrument input of your amp is 10 kohm. In general you want the input impedance of the amp to be much higher than the output impedance of your pickup.

In theory you should be okay there without a preamp, so I'm guess that's what you're doing. Once again that would point to needing to adjust the gain.


I don't really do much with audio stuff, but from engineering, if you want maximum signal transference from your pickup to your amplifier, you want the impedances to match.  In this case you have a 1 Kohm pickup impedance (Z) and a 10 Kohm amp impedance, the power (P) each uses is the current (amps, I) squared times impedance (or P=I*I*Z), so since the current is the same for each, you are putting 10 times the power you are generating into the internal pickup impedance before you get any into the amplifier, which probably isn't an issue, but that is still about 9 db of loss—but it might mean you are turning up the amp to compensate, so if you are doing that and then you do something to put more power into the pickup, you could be clipping the amplifier.

I wonder if the mounting of the pickup might be affecting this, do you notice that the double stops distortion happens at all frequencies, or more at the G string?  Also, there was some mention of the amp equalization being used, that again could drive you into clipping of the amp, since you have turned down the signal from the pickup by the amount that the equalization diminishes the signal.  I would make it flat, not turn those down—the only reason you play with EQ is if you want to CHANGE ONE PORTION OF THE FREQUENCY SPECTRUM IN RELATION TO THE OTHERS.

So you have diminished the effectivity of the pickup by 9 dB, just in the connectivity, and then you have reduced the EQ additionally (guessing another 10 dB reduction?), so then you have to compensate by turning up the amplifier. 

Amplifiers have a limit as to how much power they can provide, so when you reach that limit, either the voltage, current, or both are limited, and you get ugly results.  The when you turned down the EQ you basically made your signal have to go closer to the amp limit.

Edited by - gapbob on 10/27/2021 09:10:41

Oct 27, 2021 - 7:56:28 PM

269 posts since 6/3/2016

quote:
Originally posted by gapbob
 

I don't really do much with audio stuff, but from engineering, if you want maximum signal transference from your pickup to your amplifier, you want the impedances to match.  In this case you have a 1 Kohm pickup impedance (Z) and a 10 Kohm amp impedance, the power (P) each uses is the current (amps, I) squared times impedance (or P=I*I*Z), so since the current is the same for each, you are putting 10 times the power you are generating into the internal pickup impedance before you get any into the amplifier, which probably isn't an issue, but that is still about 9 db of loss—but it might mean you are turning up the amp to compensate, so if you are doing that and then you do something to put more power into the pickup, you could be clipping the amplifier.
 


Bob,

Don't confuse power with signal for this pre-amplification application. You are pointing out that to maximize power transfer, the output impedance of a driving device needs to match the input impedance of the load device. This is the Maximum Power Transfer Theorem attributed to Moritz von Jacobi in 1840, see for example https://electronicspani.com/maximum-power-transfer-theorem/

Here is a concise discussion of impedance matching for audio systems: http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/imped.html

And here is a quote from that site about the preamp section

On the input side, the amplifier can be made to have almost arbitrarily high input impedance, so in practice a microphone sees an impedance considerably higher than its own impedance. Although that does not optimize power transfer from the microphone, that is no longer a big issue since the amplifier can take the input voltage and convert it to a larger voltage - the term currently used is "bridging" to a larger image of the input voltage pattern.

The reason the preamplifier input impedance can and should be much higher than the output impedance of the pickup is that this is a signal amplification problem where we want to maximize the voltage at the interface. We don't care what the current and associated power is. It can be arbitrarily small. Because all we're trying to do is drive something like the input resistor of an op-amp based amplifier. That is a voltage amplifier, not a power amplifier, which is why there is a separate power amplification stage to drive speakers.  Suppose the pickup is modeled as a Thevenin equivalent voltage source V_P in series with an output resistance R_O. Suppose the input resistance of the preamp is R_I. That is a classic voltage divider, so the input voltage across the pre-amp will be V_I = V_P * (R_I / (R_O + R_I)). We want V_I = V_P, so we want R_I / (R_O + R_I) = 1. This is approximated if R_I >> R_O. Translated into plain English, the input voltage of the preamp will approximate the maximum voltage the transducer (pickup) is capable of producing at low current if the input impedance of the preamp is much greater than the output impedance of the transducer. Most of the power generated by the transducer will be dissipated in the transducer, but it doesn't matter.

If the impedances were in fact matched, the signal (voltage level) would be halved.

If we were talking about the power amplifier driving the speakers, it would be a whole other ballgame. Because there the power amplifier really is designed to operate for a very specific speaker impedance.

Is anyone still awake?

(I am an engineer, in case it's not obvious.)

Edited by - RinconMtnErnie on 10/27/2021 20:03:16

Oct 27, 2021 - 8:11:29 PM
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269 posts since 6/3/2016

I hadn't noticed it, but at that URL I referenced there is a section at the bottom called Matching Microphone to Input that is directly relevant. There is even a little calculator where you can enter the impedances of the microphone output and the preamp input. If you enter the 1000 ohm microphone (pickup) impedance and the 10000 ohm preamp impedance, there is a signal loss of 0.83 dB.

If you change the preamp impedance to 1000 ohm (matched), the signal loss is 6 dB.

That would probably acceptable, but it is certainly not optimal.

Oct 28, 2021 - 9:15:44 AM

gapbob

USA

787 posts since 4/20/2008

I would have to research more to discuss this with you, since it is getting 45 years since i did this sort of thing much, active impedances specifically.

It seems that s/n ratio is a limiting factor, since reducing the signal strength increases the noise?

Edited by - gapbob on 10/28/2021 09:20:13

Oct 28, 2021 - 1:06:51 PM

RB-1

Netherlands

41 posts since 9/28/2020

quote:
Originally posted by FiFi

I've tried connecting to both channels (1 & 2) with the EQ bands at all different settings but it doesn't improve the distortion.


No, because distortion happens in those channels already before the 'Gain' pot

There fore, it has been suggested, use channel 3 instead.

Since there is ample signal available ( 4V pp is, relatively speaking, extremely hot), you'd hardly notice any mismatch, you might even use this taming down things a bit.

But main thing, using input 3, low impedance, might cure most of your troubles.

Setting the channel volume low enough avoiding any distortion, while hammering the fiddle as loud as you can, brings you to the ideal setting for your specific situation.

Final sound level is controlled by the master pot.

And, indeed, setting all tone controls in the middle (at zero) and then experimenting with cutting or boosting the areas of your choice wil give you a substantial chance of success.

Oct 29, 2021 - 10:17:49 AM

FiFi

UK

18 posts since 5/25/2015

Hi guys. Sorry for taking a few days to get back to you, really appreciating all this help on here.

DougD - thanks for including a link to the Schertler review, looks interesting and I'm about to read that now.

RinconMtnErnie - thanks for the link to the amp manual, really helpful. I'm going to have a look at that overload light on the amp, see if it's coming on at all. The pickup I'm using has a preamp built in, so no controls there.

Considering everything that's been said on here, I think it sounds like the best thing would be for me to try channel 3 with the EQ flat (to start with anyway) and if I don't find any improvement by doing that, I now have contact details of a couple of amp experts who are based fairly close to me, I can take all my gear to them for advice.

Thanks again!

Oct 29, 2021 - 11:29:31 AM

DougD

USA

10336 posts since 12/2/2007

Fiona - If your amp is the Jam 100 then the manual Ernie linked to is not for your amp. Yous has no overload lights, for example - its a completely different amp.
Presumably your amp came with a manual, but if not, there's a link to an online version on this page: djangobooks.com/Item/sr-jam-10...s-natural
This may be more than you need to know, but in that review he refers to the inputs on channel 3 as "Lo" and "High" impedance, but in the manual it seems more likely that "Lo" and "High" refer to the gain (sensitivity) of the inputs (even though the description on the Djangobooks page refers to impedance - go figure)!
I would try the "Lo" input first, and if its still distorting, try the "High." You won't really hurt anything, and hopefully this will solve your problem in about five minutes.

Oct 29, 2021 - 12:30:14 PM

DougD

USA

10336 posts since 12/2/2007

Whoops - Ignore that last part Fiona. Try the "Hi" input first, then the "Low." As you can see, I find that labelling confusing!

Oct 29, 2021 - 1:58:14 PM

FiFi

UK

18 posts since 5/25/2015

Hi DougD,

You know, when I read Ernie's comment I was thinking I'd never noticed any light coming on in the past ... thanks so much for pointing this out, I didn't get a manual with the amp but I've now downloaded it from your link. I'm going to try your suggestion tomorrow, fingers crossed it works! :-) Thanks again!

Oct 29, 2021 - 8:28:12 PM

DougD

USA

10336 posts since 12/2/2007

Fifi - One more thing. I don't know in what language your manual was originally written, but it apparently was not English! The section covering the channel 3 inputs is particularly confused. You might also want to look at the manual for the Jam 150, which has a similar layout and controls. Its available here: manualslib.com/products/Schert...1104.html
It makes it clearer that you can use either of the two inputs, or both (for a stereo keyboard, etc.).

Oct 31, 2021 - 5:00:56 AM

FiFi

UK

18 posts since 5/25/2015

DougD - thanks for that further link.
I spent some time yesterday trying out the third channel but, unfortunately, no luck. :-( Same problem, sounds fine until I turn up the master volume to roughly 'five to' then I get this 'distorted' sound. The 'gain/volume' knob on the channel was only up slightly (barely '25 to'), I need to have that up, even slightly, to get any sound coming out the amp. I tried both hi and lo inputs.
Then, I also connected my fiddle up to a friends amp that he uses for his acoustic guitar and ... same thing happens! Sound starts to distort at a certain volume! So, it sounds like the problem is with the pickup or fiddle rather than the amp! I've spoken to my luthier and he suspects it's something to do with the pickup. (Can't remember if I mentioned that I've tried playing my fiddle next to a mic and all sounds fine coming out of the PA.) So ... the next step is my fiddle and pickup are going to my luthier next weekend. Hopefully he'll get to the bottom of it.
Thanks again all!

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