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Icom 756: Increasing the TX and RX Audio Bandwidth

Posted: Wed Aug 07, 2013 3:53 am
Hello everybody!

In case someone wants to increase the audio bandwidth on the Icom 756 (non-Pro), I've written a short report explaining the changes I've made to my rig.

I would post the link here, but the system does not allow me to do that yet. Will do as soon as I can post url's!

Anyway, with this mod you can get more than 5 kHz of SSB / AM TX bandwidth. The patch is pretty clean and it has been working nicely for several months now.


Posted: Wed Aug 07, 2013 3:55 am
Now I can, so here it is! ... h_mods.pdf

Comments and suggestions are welcome !!!


Posted: Wed Aug 07, 2013 10:50 am
by Voodoo Guru
Howdy Daniel,
Welcome to the Forum!!!

EXCELLENT Article Daniel!
That is what were talking about, true Experimentation!
That is incredible research and modification.
Thank You for posting it on the Forum.
This will help many people to have an alternative transceiver to play with eSSB.

We are honored to have you as a Member.
Thank You again for your contribution to this fine facet of Amateur Radio.

This Thread will be Sticky Posted to the top of this Topic.

Take Care,

Posted: Fri Nov 15, 2013 6:24 pm
by K9JPD
Is anyone out there capable and experienced to do this. My Icom is my only radio so a decent turnaround would be nice. And what would be a price on this job. I'm good at soldering just not good with soldering at the critical scale. I could just send u the Dsp board if that's all that's needed for the modification. Thanks! Very interested in equipping my icon with this.

Posted: Mon Nov 18, 2013 9:22 am

I'll be glad to help, the mod takes only a few minutes if you know what components to change, but I'm afraid it will be way too overkill to send the board to Spain and back to USA just to change a few passives.

I'm pretty sure you can find some local experienced technician that, after viewing the document, can perform the mod quickly and for little money. I'll be glad to help him if there is something unclear.

Best regards,

Posted: Fri Dec 06, 2013 11:33 pm
by K9JPD
I appreciate it. I will just be in the search of someone to do this modification for me. Id like to have it done to see what it sounds like.

Posted: Wed Dec 18, 2013 2:12 am
by !KANT
Keep us updated 'JPD, would really enjoying hearing an audio file of your successful modification.

Posted: Wed Mar 04, 2015 1:41 am
by toxicooze
Hello voodoo audio crew

I've got an Icom 756(non pro) to mod to get the voodoo audio.

My current set up is

PR 40
W2IHY 8 Band EQ
EQ Plus
Ranger 2950 DX

Sounds good and get good reports but need/addicted to the TX/RX Bandwidth

for that voodoo audio sound.

New set up will be to start with (hope config is right)

PR 40
Icom 756

I'm connecting the ibox audio out cable via the back of the Icom through the

ACC1 as suggested here :).

I've got the DEQ2496 set up, thanks to all the info here and on the web but

could someone please give me basic starting settings for the MIC2200 if


I've brought some Starquad Canare cable and XLR connectors as I'm wanting to

make my own cable lengths with .001uf capacitors as suggested but not sure

on the exact .001uf capacitor to use and if you solder one capacitor

for every pin/shield. Could someone provide me the info with the right capacitor to use, a pic or diagram of the XLR cable connector made would be appreciated.

Any info or help would be appreciated and gratefully received.


Posted: Wed Mar 04, 2015 4:19 pm
Hello Bob,

Just a tip on the ACC1 audio: The rig, as it is, will give you a low audio fidelity through this connector, as it is intended for digital-like signals only. On the MAIN UNIT board there is an opamp (IC503b) which works near saturation and also has some high-pass filtering on it. I've changed a few components on my rig to get high-fidelity audio though ACC1 and this required to remove the whole board form the rig and change two or three SMD resistors and capacitors... It's not and easy mod... I suggest you do it only if you're comfortable with this level of hacking.

Anyway, with the mod I described you'll get quite some bandwidth just using the front mic connector. The ACC1 audio is better, yes, but not so noticeable.

By the way, if you transmit on AM, I suggest to increase R426 to at least 4.7kOkm (in order to get similar TX audio gains on AM and SSB) and to reduce R449 to 7.8KOhm or so (to get full 100% AM modulation depth without reaching the opamp positive output rail). Both are on the DSP board. Otherwise, if your rig is as mine, you won't get a decent audio level on your AM transmission.

Good luck!

Posted: Sat Mar 07, 2015 12:40 am
by ECC82
EA3HRU wrote:Now I can, so here it is! ... h_Mods.pdf

Comments and suggestions are welcome !!!
Link no longer available?

Posted: Sat Mar 07, 2015 1:16 am
Hummmm. It appears that the whole server is down.... I don't have access to other stable web server, so anyone needing it, send me an email to daniel_at_z-ha-dum_dot_dyndns_dot_org . Te file is free to share, so if anyone can post it on the web, feel free to do so.


Posted: Sat Mar 07, 2015 10:18 pm
by toxicooze
Hi Daniel

The modifications you have done on your Icom 756 require a lot of skill and knowledge. Unfortunately I myself can't do what you can do. So I'm going to have to buy another radio more set up for ESSB for myself because that last hacking mod on the main board for the acc1 fidelity audio is to complex for me and here in New Zealand not a lot of people are on hand to do what's required.

Thanks for all the info, it will most probably help a lot of other operators that can do these mods to get the voodoo in there audio with the Icom 756..



Posted: Mon Mar 09, 2015 7:26 am
by Voodoo Guru
Howdy Daniel,

We've added your PDF Mod to our server and changed the link in your original Post.

So to all of those who were looking for the mod, the link in the original post above now works.

It is also posted here for your convenience... ... h_mods.pdf

Take Care,

Posted: Sun May 10, 2015 11:54 pm
by iz2oby
Good morning to all of Italy.
Daniel a question: what is the value of the two capacitors in series at both ends of the filter FL-33.
Thanks and sorry for my poor english.

Posted: Mon May 11, 2015 1:34 am

I don't remember exactly, but it was between 10nF and 100nF. I only used them to block the DC component coming from the radio, otherwise the filter shorts it out to ground. Nothing fancy like impedance matching or so...

Best regards,

Posted: Mon May 11, 2015 5:05 am
by iz2oby
Thank you, Daniel, and congratulations again for your work on the IC-756.
Suggest to not connect the ACC1 and use only the front microphone jack without further mods?

Posted: Mon May 11, 2015 6:06 am

You cannot put decent analog audio through ACC without changing a few components on the bottom of the Main Unit board. This requires removing the board from the radio. This is independent of increasing the TX audio bandwidth, you still can use the front mic and go wide...

Good luck,

Posted: Sat Jul 08, 2017 5:41 am
by kg6pqq
I havent scrubbed the schematics as of yet but is the Non Pro and the Pro 3 layout that different or could this mod apply?

Posted: Sun Jul 09, 2017 11:48 am

I don't really know, check on the schematic if they're based on the same filter chip. If they're you may have a chance!

Best regards,

Posted: Sun Jun 10, 2018 12:45 am
by ew1mm
To EA3HRU Daniel.
I am making some mods on IC756 now.

You wrote (Wed Mar 04, 2015 7:19 pm ) this:
Just a tip on the ACC1 audio: The rig, as it is, will give you a low audio fidelity through this connector, as it is intended for digital-like signals only.
On the MAIN UNIT board there is an opamp (IC503b) which works near saturation and also has some high-pass filtering on it. I've changed a few components on my rig to get high-fidelity

So, what did you do?
What radio parts did you change around IC503b opamp?
Can you give more info?
With Best Regards,
Gary EW1MM.