Help! EHX Electric Mistress NOISE with Free the Tone Overdrive

[tj7]

https://soundcloud.com/matthew-h-slaughter/ste-003

That's the sound I'm getting when I have my distortion pedal running into my EHX Flanger, whether the distortion pedal is engaged or not.
These two pedals do not get along - and I require the distortion to be before the flanger in my chain.

This sound persists even when both pedals are running via battery, and I have tried a buffer pedal to no avail...

Flanger: 
http://www.metzgerralf.de/elekt/stomp/mistress/electric-mistress-v5.shtml

Overdrive:
http://www.freethetone.com/en/products/effects_custom/sov-2_overdrive.html

I have tried a buffer pedal before and after the flanger - does absolutely nothing.

Really could use a fix... or an idea of how to fix so I can take it to my local tech to look at...!

[Scruffie]

Quick read of the Overdrive page and it appears it uses a charge pump type set up, the noise you're getting is called heterodyning when 2 clocks interact like that.

Solution, nothing springing to mind.

[thelonious]

The easiest fix is to replace the overdrive with something that will run at 18V power instead of using an internal charge pump to turn 9V into ~18V. But if you are really sold on both pedals and want to keep them, you might be able to minimize or even eliminate the noise by externally clocking the charge pump. It worked for me with a charge pumped overdrive and a Timeline on high 'Grit' settings. You need to get the heterodyning above or below the range of human hearing. I'll dig into my archives and look at exactly what I did... it was a while ago.

[slacker]

Go back to Free the Tone and tell them that their "...highly efficient filter....to prevent the converter's oscillator from interfering with the audio signal." isn't working and see what solution they can offer.

[anotherjim]

Quick read of the Overdrive page and it appears it uses a charge pump type set up, the noise you're getting is called heterodyning when 2 clocks interact like that.

Solution, nothing springing to mind.

+1
There's a double whammy because the EM has no low-pass input filtering, the delay line sampling rate is so high, it doesn't need it in normal circumstances. In fact, it has pre-emphasis that boosts HF going in! There's a couple of easy places in the EM circuit that some low pass filtering could be added, for the cost of 2 capacitors.

A possible cure is to use a DI isolation box - the cheap kind with an audio transformer inside (Samson, Behringer?) but don't use the ground lift function. The high frequency noise from the Overdrive supply isn't going to make it through an audio transformer. Or build filtering into a buffer pedal.

[thelonious]

Go back to Free the Tone and tell them that their "...highly efficient filter....to prevent the converter's oscillator from interfering with the audio signal." isn't working and see what solution they can offer.

Agree. For the prices they're charging and the things they're claiming in the description, it ought to be dead quiet, shine your shoes, and make your guitar poo diamonds when you strum. I think their customer service will have an interest in making you happy (hopefully).
___
Edit: Even if anotherjim and scruffie are right, and the EM is mainly at fault, FTT will still probably have some informed suggestions for you. It's unlikely that you're the first customer to face this issue.

[Scruffie]

Quick read of the Overdrive page and it appears it uses a charge pump type set up, the noise you're getting is called heterodyning when 2 clocks interact like that.

Solution, nothing springing to mind.

+1
There's a double whammy because the EM has no low-pass input filtering, the delay line sampling rate is so high, it doesn't need it in normal circumstances. In fact, it has pre-emphasis that boosts HF going in! There's a couple of easy places in the EM circuit that some low pass filtering could be added, for the cost of 2 capacitors.

A possible cure is to use a DI isolation box - the cheap kind with an audio transformer inside (Samson, Behringer?) but don't use the ground lift function. The high frequency noise from the Overdrive supply isn't going to make it through an audio transformer. Or build filtering into a buffer pedal.

You just reminded me! I breadboarded this version of the mistress a long time ago and had the noise as I was using a charge pump and all that was needed was the 1nF cap between pins 6 & 7 of the 4558 that was found in later versions.

So give that a go, it'll slightly change the mistress sound but, worth it i'd say.

[tj7]

Thanks, all!

First up, I bought the FTT pedal "as new" on ebay for $250 – it literally was brand new, still in the box and shiny – and works a treat in all other circumstances.  But I can't take it back to the manufacturer as I technically bought it "second hand".

Secondly, any fix can't involve mucking around with the Mistress.  I have gone through 3 vintage Mistresses to try and resolve the old volume drop issue and every single time the tone is slightly changed and it's just not right for me.  I can't get it modded.  At all.  It sounds perfect the way it is!

I do have a DI box but it's a Countryman type 85 – I wonder if that will do the job...?  Will have to try...  or any suggestions on a buffer pedal that has filtering built in?
I am not a techie and can't build pedals or do more than a basic solder, so I'm limited to using a guy to do it for me, or buying whatever I need prebuilt...

[anotherjim]

After looking at the market, all the transformer DI boxes have fixed ground isolation. You don't want that or your guitar won't be grounded to the amp. It would be simple to solder a wire between in and out jack shields in something cheap like the Behringer Hum Destroyer.

Maybe you should take your pedals to the guitar store and try them with any pedals they have that can provide clean buffering? Modern designs are likely to include filtering of out of band signals.

As to modding the Mistress, I'm surprised to hear that the 1nF cap mod mentioned by thelonius changes the tone. It adds filtering only above 23Khz!

[Scruffie]

I'm only going from a year old memory but the 1nF might have... rounded things off a bit, just a hair? The filtering in the mistress is extremely light so I guess that even minor extra filtering might be noticeable OR my memory is just mistaken. It's also possible that just the removal of the heterodyning gave an apparent change in tone.

However, for a 2 minute job just to tack a cap on to the solder side of the board, i'd give it a go, if you don't like it, just snip it off. I really think it'll be your best option if you want to continue using both.

Who knows, you might even find you prefer it!

I don't think a buffer of any kind is the solution here, unless it happens it rolls of 25kHz (using the LT1054 as a rough estimate of what frequency the OD clock is running at).

[Govmnt_Lacky]

...all that was needed was the 1nF cap between pins 6 & 7 of the 4558 that was found in later versions.

Did they sever the direct connection between Pins 6 and 7 when they did this? According to the schematic, those pins on the 4558 are shorted. 

[Scruffie]

Ah yes, shouldn't have assumed, it's pins 6 & 7 on the 9V mistress but the 4558 package is flipped for the 18V model so it's 1 & 2. So parallel to the 6k8 resistor.

[thelonious]

As to modding the Mistress, I'm surprised to hear that the 1nF cap mod mentioned by thelonius changes the tone. It adds filtering only above 23Khz!

Actually I think scruffie said that, but re the frequency: ~23kHz is the -3dB frequency of a 6dB/oct filter, right? So it would actually start shaving the highs at a lower frequency than that. Whether or not it's a perceptible difference at, say, 12kHz, I don't know. Possibly by someone with better ears than me.

Edit: graph

[tj7]

If this is something I can do with only very basic soldering ability, and it is definitely a reversable process, I'm willing to give it a go.
However, I need this to be totally idiot proofed for me.
If you have time, if I send you pics of the board could you show me exactly what I need to do and where to do it?
I also need the exact part to find online - I don't even know what a 1nF cap looks like (will google now...)
Like - I need really clear, "electronics for dummies" steps for this process....

I'm only going from a year old memory but the 1nF might have... rounded things off a bit, just a hair? The filtering in the mistress is extremely light so I guess that even minor extra filtering might be noticeable OR my memory is just mistaken. It's also possible that just the removal of the heterodyning gave an apparent change in tone.

However, for a 2 minute job just to tack a cap on to the solder side of the board, i'd give it a go, if you don't like it, just snip it off. I really think it'll be your best option if you want to continue using both.

Who knows, you might even find you prefer it!

I don't think a buffer of any kind is the solution here, unless it happens it rolls of 25kHz (using the LT1054 as a rough estimate of what frequency the OD clock is running at).

[Scruffie]

Yeah send me a photo and i'll draw it on for you. Don't worry it's very easy.

The .001mF cap from here should suffice http://smallbear-electronics.mybigcommerce.com/topmay-tmc05-001-mf-1-mf/ unless you have a preferred supplier.

[tj7]

Thanks - will do!
Can you please tell me the value of the cap I need to order?
I am in Australia and will be ordering from here:

http://www.jaycar.com.au/productResults.asp?MID=1&SSUBID=638&SUBCATID=967&keyform=CAT2#1

If you can point out the right one for me, I'd appreciate it!
Ben

Yeah send me a photo and i'll draw it on for you. Don't worry it's very easy.

The .001mF cap from here should suffice http://smallbear-electronics.mybigcommerce.com/topmay-tmc05-001-mf-1-mf/ unless you have a preferred supplier.

[Scruffie]

Here ya go http://www.jaycar.com.au/productView.asp?ID=RC5336&form=CAT2&SUBCATID=967#1

[duck_arse]

tj7 - send me a pm, I might be able to help you out with a cap or two.

[tj7]

Here ya go http://www.jaycar.com.au/productView.asp?ID=RC5336&form=CAT2&SUBCATID=967#1

Thanks!

[tj7]

Yeah send me a photo and i'll draw it on for you. Don't worry it's very easy.

The .001mF cap from here should suffice http://smallbear-electronics.mybigcommerce.com/topmay-tmc05-001-mf-1-mf/ unless you have a preferred supplier.

Here you go!
http://tinypic.com/r/167uhw3/8
I have PM'd you my email address...