freestompboxes.org

Hi all.

Can someone explain to me exactly what the "FM" mode does, (from an electronic perspective) and how it could be wired to a standard TS clone.

I'm building a TS clone for a friend of mine, and would like to include this feature. I have a limited understanding in electronic theory however, so can't quite figure out what is going on there by looking at the schematics, due to the complications of the other modes and the boost switch in the same vicinity.

I understand what it does sonically, since I own a FD2, but can't work out what it is doing electrically.

Would someone here, who has some info on it be willing to share?

Thanks.

Bendo031 wrote:Would someone here, who has some info on it be willing to share?

viewtopic.php?f=13&t=10629&p=116127

Hi Modman.

Thanks for that. I've actually seen that link before. Doesn't make any reference to what the FM mode does.

All the projects etc that I have seen have the FM switch detailed in the schematics, and I would be able to wire it, but I still have not come across an explanation of what exactly the FM switch actually does, from an electronic perspective. I know that it reduces the amount of mids, but would like some technical details.

Anybody?

It's just feedback. Similar to what tube amps have sometimes from the transformer tap.

switching a bigger cap in makes it seem more full & less midrangey ?

indyguitarist wrote:It's just feedback. Similar to what tube amps have sometimes from the transformer tap.

Thanks Indyguitarist.

That makes sense now, re-looking at the schematic. So the signal is actually being sent back from pin 7 of the op-amp to pin 2 via a couple of caps and resistors? Interesting.

Seems simple enough to implement on any TS clone.

Is Indyguitarist Brian Wampler btw? Wow if it is! Thanks for the response. What an honor.

Hi there..

I am new to post on this forum, though having enjoyed a lot of the great information available on the site. Thanks to you all..! I am going to give my view on this subject, hoping to give a bit back..

The "Flat Mids" function actually seems to be some kind of feed-forward as opposed to the statement that it was feedback, like in amplifiers..? What it does is to blend some of the "full-range" signal, taken prior to the 1K/0,22uF low-pass (723Hz) network, virtually straight into the pedals output section. It works well, giving a nice sound that seems much more neutral and "open" than the standard mode. A major drawback, though, is that the tonecontrol doesn't do much at all in the flat mids mode. Also, the circuit seems to lose some gain, suggesting that the flat mids might also serve as a sort of feedback after all.. I am not 100% sure..

I have for some time been trying to build my own "signature overdrive," based on the screamer pcb sold by Musikding.de and started messing around with the 1K/0,22uF network that, in combination with the 4K7/0,047uF in the feedback loop, creates the TS-style midrange hump centered at 723Hz. As I saw the tread on the Rockbox boiling point elsewhere on this site, I discovered, using muzique.com's online RC-filter calculator, that the combination I liked the most filtered both the high- and the low-end at exactly the same frequencies as Rockbox do.

My point is that Fulltones Flat mids function simply counteracts the low-pass filter at the expense of the tonecontrol and quite a bit of gain. It seems a bit, well, crude, if you ask me.. Adjusting the low-pass filter to your likings can create the exact same effect, but leave the tonecontrol functioning and with a lot more gain AND output from the circuit. It also seems to be the final step towards "modding a Tubescreamer to not at all sound like a Tubescreamer anymore.."

Best regards,
Morten Lyse, Denmark.

MrLyse wrote:Hi there..

I am new to post on this forum, though having enjoyed a lot of the great information available on the site. Thanks to you all..! I am going to give my view on this subject, hoping to give a bit back..

The "Flat Mids" function actually seems to be some kind of feed-forward as opposed to the statement that it was feedback, like in amplifiers..? What it does is to blend some of the "full-range" signal, taken prior to the 1K/0,22uF low-pass (723Hz) network, virtually straight into the pedals output section. It works well, giving a nice sound that seems much more neutral and "open" than the standard mode. A major drawback, though, is that the tonecontrol doesn't do much at all in the flat mids mode. Also, the circuit seems to lose some gain, suggesting that the flat mids might also serve as a sort of feedback after all.. I am not 100% sure..

I have for some time been trying to build my own "signature overdrive," based on the screamer pcb sold by Musikding.de and started messing around with the 1K/0,22uF network that, in combination with the 4K7/0,047uF in the feedback loop, creates the TS-style midrange hump centered at 723Hz. As I saw the tread on the Rockbox boiling point elsewhere on this site, I discovered, using muzique.com's online RC-filter calculator, that the combination I liked the most filtered both the high- and the low-end at exactly the same frequencies as Rockbox do.

My point is that Fulltones Flat mids function simply counteracts the low-pass filter at the expense of the tonecontrol and quite a bit of gain. It seems a bit, well, crude, if you ask me.. Adjusting the low-pass filter to your likings can create the exact same effect, but leave the tonecontrol functioning and with a lot more gain AND output from the circuit. It also seems to be the final step towards "modding a Tubescreamer to not at all sound like a Tubescreamer anymore.."

Best regards,
Morten Lyse, Denmark.

Hi Morten.

Thanks for that post.

Definitely the most useful reply I've yet had.

Not being educated in electronics, I have a very limited understanding of how these things work. All of my knowledge so far has been gained by modifying and building circuits, by using FX kits etc. I have built lots of guitar effects and a few small tube amps, but what I'm doing is normally based off other peoples information, rather than my own designs, or understanding of electronics. Obviously, along the way, I've learned some basic principles, and gained some understanding, but can't always figure out what I am looking at in a schematic.

I would think you are right, in that the FM is a feed-forward device. I own a FD2, and would concur with your observations. I'm not sure I noticed any significant gain reduction, but I did notice the tone control issue that you spoke about. Since the guy I'm building the pedal for (a friend of mine) wants a FD2 clone, I will include this feature, despite not finding it really that useful.

In a few TS clones that I've built for myself (and others) I have gone about removing the "mid hump" in similar ways to you, by increasing the value of the 47n cap to a 100n. Common mod, I know, but it sounds good, although I don't think it really makes the TS "mid hump" signature sound severely diminished. I know the FD2 has a 100n cap there anyway.

I'm attempting to come up with a way of wiring the FM switch, as a separate switch to the clip toggle. Looks simple enough, but just needed some advice from someone with a little more knowledge than me about these things. Your post has helped me better understand how I could go about this.

Many Thanks.

You are welcome.

If you want to dig a little deeper into "what goes where" in the tube-screamer circuit, don't miss R.G. Keens excellent article on this matter:

http://www.geofex.com/article_folders/t ... sxtech.htm

I, by the way, also use 4K7/100nF in the basscut network, and then 1K5/100nF in the treblecut circuit, after the clipping section. Not for everyone, I'm sure. My friends Rivera doesn't sound very good with this configuration, but its a match made in heaven with my Matchless HC-30 clone.

Best regards.
Morten Lyse