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tech / rec.audio.pro / Re: Pultec EQP clone problem

SubjectAuthor
* Re: Pultec EQP clone problemHot Jam Studios
`* Re: Pultec EQP clone problemScott Dorsey
 `- Re: Pultec EQP clone problempalli...@gmail.com

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Re: Pultec EQP clone problem

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Subject: Re: Pultec EQP clone problem
From: hotjamst...@gmail.com (Hot Jam Studios)
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 by: Hot Jam Studios - Sun, 12 Nov 2023 14:35 UTC

On Wednesday, February 3, 2021 at 11:40:16 AM UTC, gareth magennis wrote:
> On Wednesday, 3 February 2021 at 01:48:00 UTC, palli...@gmail.com wrote:
> > gareth magennis is super annoying :
> >
> > =========================
> > >
> > > How can I force this motorboating to happen on my bench, so I can deal with it?
> > > I've not seen it yet.
> > ** FFS !!!
> >
> > How will you ever know if you have fixed it ?????
> >
> > Rule one of troubleshooting: " Observe the Fault "
> >
> >
> >
> > ..... Phil
> Ok, I think I may have got to the bottom of this.
>
> I found this, which I believe is the issue and its solution.
> https://www.gearslutz.com/board/so-much-gear-so-little-time/1104044-warm-eqp-wa-modifications.html
>
>
> There, it states the mod is a 25pf in series with 100 ohms, which makes much more sense.
> I fitted it and, of course, it doesn't drop the level by 12dB.
>
> I did measure things before finding this, and the output inpedence of the circuit is around 360 ohms, in the method described by Mike.
> Thanks MIke.
> I also tried to measure the low frequency rolloff, which was way below 10 Hz.
> Thanks, Phil.
>
> Maybe there never was any "motorboating", this notion came from the customer in his email to the company.
> I never found any, despite adding caps on the output like Mike suggested.

Hi,
I am having the same issue that is reported in this conversation. I have a Warm Audio EQP-WA with a pretty low serial number ( less than 400). The motor boating happens for me when running the XLR ins/outs to a patchbay. It is a constant ticking sound that I could understand why it might be referred to as 'motor boating'. This happens a couple of seconds after the unit is powered on. If I bypass my patchbay and run from the TRS ins/outs the motor boating doesn't occur from powering the unit however when extreme boost or cut is applied a similar 'motor boating' sound does occurs but it does seem to match the relevant frequency boosted or cut and I can understand why it might be described and an oscillation. I am happy to shoot a video showcasing the setup and issues to share, however I am more interested its little more information of on the fix above to be able to relably use this unit.

Re: Pultec EQP clone problem

<uitg6c$9r7$1@panix2.panix.com>

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From: klu...@panix.com (Scott Dorsey)
Newsgroups: rec.audio.pro
Subject: Re: Pultec EQP clone problem
Date: 13 Nov 2023 15:43:08 -0000
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 by: Scott Dorsey - Mon, 13 Nov 2023 15:43 UTC

Hot Jam Studios <hotjamstudios@gmail.com> wrote:
>On Wednesday, February 3, 2021 at 11:40:16=E2=80=AFAM UTC, gareth magennis =
>wrote:
>> Ok, I think I may have got to the bottom of this.=20
>>=20
>> I found this, which I believe is the issue and its solution.=20
>> https://www.gearslutz.com/board/so-much-gear-so-little-time/1104044-warm-=
>eqp-wa-modifications.html=20
>>=20
>>=20
>> There, it states the mod is a 25pf in series with 100 ohms, which makes m=
>uch more sense.=20
>> I fitted it and, of course, it doesn't drop the level by 12dB.=20
>>=20
>> I did measure things before finding this, and the output inpedence of the=
> circuit is around 360 ohms, in the method described by Mike.=20
>> Thanks MIke.=20
>> I also tried to measure the low frequency rolloff, which was way below 10=
> Hz.=20
>> Thanks, Phil.=20
>>=20
>> Maybe there never was any "motorboating", this notion came from the custo=
>mer in his email to the company.=20
>> I never found any, despite adding caps on the output like Mike suggested.
>
>
>Hi,=20
>I am having the same issue that is reported in this conversation. I have a =
>Warm Audio EQP-WA with a pretty low serial number ( less than 400). The mot=
>or boating happens for me when running the XLR ins/outs to a patchbay. It i=
>s a constant ticking sound that I could understand why it might be referred=
> to as 'motor boating'. This happens a couple of seconds after the unit is =
>powered on. If I bypass my patchbay and run from the TRS ins/outs the motor=
> boating doesn't occur from powering the unit however when extreme boost or=
> cut is applied a similar 'motor boating' sound does occurs but it does see=
>m to match the relevant frequency boosted or cut and I can understand why i=
>t might be described and an oscillation. I am happy to shoot a video showca=
>sing the setup and issues to share, however I am more interested its little=
> more information of on the fix above to be able to relably use this unit.

The thing is motorboating. It's motorboating because the poles are on the
wrong side of the plane; when a capacitive load is put on the output, the
feedback around the output amplifier is delayed sufficiently that it goes
from amplifying to oscillating. This is a design problem.

Contact Chad Kelly at Warm Audio and ask them to do the factory mod for you
like the guy on Gearslutz details. It looks like they are adding two RC
constants to the thing and the important one is probably the one he details
on R10. But without a schematic it's hard to tell the details.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Re: Pultec EQP clone problem

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Subject: Re: Pultec EQP clone problem
From: palliso...@gmail.com (palli...@gmail.com)
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 by: palli...@gmail.com - Tue, 14 Nov 2023 01:35 UTC

Scott Dorsey wrote:
=========================
>
> >I am having the same issue that is reported in this conversation. I have a =
> >Warm Audio EQP-WA with a pretty low serial number ( less than 400). The mot=
> >or boating happens for me when running the XLR ins/outs to a patchbay. It i=
> >s a constant ticking sound that I could understand why it might be referred=
> > to as 'motor boating'. This happens a couple of seconds after the unit is =
> >powered on. If I bypass my patchbay and run from the TRS ins/outs the motor=
> > boating doesn't occur from powering the unit however when extreme boost or=
> > cut is applied a similar 'motor boating' sound does occurs but it does see=
> >m to match the relevant frequency boosted or cut and I can understand why i=
> >t might be described and an oscillation. I am happy to shoot a video showca=
> >sing the setup and issues to share, however I am more interested its little> > more information of on the fix above to be able to relably use this unit.

-------------------------------------------------------
> The thing is motorboating. It's motorboating because the poles are on the
> wrong side of the plane; when a capacitive load is put on the output, the
> feedback around the output amplifier is delayed sufficiently that it goes
> from amplifying to oscillating. This is a design problem.
>

** See example of genuine tube amp amp" motorboating".
https://www.youtube.com/watch?v=iDeXlQIvqco

Note the sound is low frequency, continuous and much like that of a small IC engine buried inside the hull of a boat.

Pultec EQP clones have a different problem, caused by high frequency instability due to the elaborate transformer coupled feedback arrangement used in the balanced line output stage. The clicking sound heard is the result of intermittent supersonic frequencies at the output usually know as "squegging" a contraction of the term "self quenching". As you say, the stage is oscillating due to negative feedback becoming positive under certain load conditions, likely excess capacitance to ground.
The output transformer is the culprit and the fix will involve taming the high frequency gain of the tube stages a tad.

...... Phil


tech / rec.audio.pro / Re: Pultec EQP clone problem

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