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General Discussions => Drawing Board => Topic started by: leadbreath on February 06, 2016, 04:38:22 AM

Title: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 06, 2016, 04:38:22 AM
Hi Folks

After many years on this forum gleaning as much info and help that I can possibly suck into my drug addled brain I can finally give something back to this forum.

I have managed to get the Schematics for the legendary  preamp manufactured around 1970 in Hayes Middlesex.(Numbers 1 to 5)Mk 1

Sorry to be so cryptic but as we all know many attempts to bring this pre to life have been cut short by the powers that be.

I have the Desk block diagram and schemos for the Input card , Output/line card and the Hi and Lo Eq's.
The EQ Card schemos are quite grainy and some of the values cant be seen but I'm sure we can figure it out.

I provided the output drawing to Brian Sowter and he has already wound an output tranny, so its looking good.
I'm just waiting for some free time  then I will knock one of these bad boys up, cant wait!!!

So what do you think shall I upload or not???
Title: Re: Hayes Middlesex Preamp
Post by: ruffrecords on February 06, 2016, 06:36:32 AM
Upload!

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: ding on February 06, 2016, 10:06:21 AM
Yes, please!!!!!  No comp/limit?
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 07, 2016, 03:48:18 AM
No comp/limit  at the moment unfortunately, my source has said that he has a loads of drawings from that company but only managed to find the ones I mentioned so far in relation to the "numbers" desk.

Will update when/if he find more info.

OK so it doesn't seem like its a problem to upload these drawings so keep watching this space!!
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on February 07, 2016, 07:33:14 AM
OK so it doesn't seem like its a problem to upload these drawings so keep watching this space!!

Best place to upload them would be to the tech docs folder.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 07, 2016, 08:58:09 AM
Hey Ian,

Can do, I'll also attach some copies to this post as would like to confirm some of the missing values as mentioned before.
Title: Re: Hayes Middlesex Pre/Channel
Post by: ding on February 07, 2016, 02:41:29 PM
No comp/limit  at the moment unfortunately, my source has said that he has a loads of drawings from that company but only managed to find the ones I mentioned so far in relation to the "numbers" desk.

Will update when/if he find more info.

OK so it doesn't seem like its a problem to upload these drawings so keep watching this space!!

Too bad, it would be amazing to have a channel of this with pre/eq/comp-limit. Where is the docs folder?
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on February 07, 2016, 02:59:58 PM

Too bad, it would be amazing to have a channel of this with pre/eq/comp-limit. Where is the docs folder?

http://groupdiy.com/index.php?board=19.0 (http://groupdiy.com/index.php?board=19.0)

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:11:58 PM
Right here we go:

Input Amp



Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:13:07 PM
Output amp:
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:14:04 PM
Treble Circuit
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:14:41 PM
Bass Circuit
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:15:17 PM
Desk block diagram
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:16:06 PM
Limiter info:
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:20:44 PM
Ian I had a look at posting these on the Docs folder but I'm not too sure about it as I don't want to call it by its real name?
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 08, 2016, 07:26:18 PM
Damn! just  realized I'm missing the second part of the Desk block diagram, will look for it and post when  found.

Now I'm off to bed, its been a looong day.
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 10, 2016, 01:47:36 PM
Ian

Don't know if you've looked at the schematics, I was just wondering what your thoughts are on the Hi/Lo Board  Gain switches?

It seems like it was a separate  board that they utilized with all the resistors preassembled?

Not sure of what value range we should be looking at, what do you think??

Mick
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on February 11, 2016, 07:02:14 AM
Ian

Don't know if you've looked at the schematics, I was just wondering what your thoughts are on the Hi/Lo Board  Gain switches?

It seems like it was a separate  board that they utilized with all the resistors preassembled?

Not sure of what value range we should be looking at, what do you think??

Mick

Both bass and treble EQs use the same basic topology.  It is basically an inverting op amp. The input arm is the 20K R1 in parallel with the left hand string of resistors on the switch and the feedback arm is the right hand string of resistors on the switch.  The gain of the stage is equal to the feedback resistance divided by the input resistance.

The bass circuit uses a 1,93H inductor in series with a 750 ohm resistor as the frequency selective element which will produce a shelving response. The treble circuit uses a 100mH inductor in series with a 10nF capacitor and a 1K resistor as the frequency selective element which will produce a bell shaped response centred around 5KHz.

To make things easy, let's assume that when the controls are flat the gain is unity. We can make the right hand switch resistor of the  input arm arbitrarily large so we can assume when the controls are flat the input resistor is 20K. Therefore the sum of the switch resistors in the feedback path must also be 20K to achieve unity gain. From then on it is just a matter of calculating the switch resistor total at each position to get the 2dB steps required. Apart from that, all we know is that the max boost and cut is 10dB.

For 10dB boost, the input arm needs to be reduced to 6.3K. To reduce 20K to 6.3K needs a resistor in parallel of 9.2K. This 9.2K is made up of the left most switch resistor in series with the inductor dc resistance and the 750 ohms resistor. Let us assume the dc resistance of the inductor is 250 ohms which means the switch resistor is 8K2 ( a nice standard value available in those days).

Now repeat this process remembering the next switch resistor value will be whatever needs to be added to the 8K2 to get the next value. So for 8dB boost the input resistor arm needs to be 8K which means our total switch resistance needs to be 13K3. So the next resistor in the switch string need to be:

13K - 8K2 - 250 -750 = 3K8 - they probably used a 3K9 resistor

Use a similar process to calculate the remaining switch resistor values.

Cheers

Ian

Edit: I have no idea how I managed to get those paragraphs shuffled. Hopefully they are now in a sensible order.
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 12, 2016, 07:40:53 PM
Awesome Ian, thanks that makes more sense now.

I've also  realised that we are missing another important piece - The drawing for the Band pass filter?
I've heard that this is an important part of the sound of this desk, not sure what frequencies we're talking of here but I can try and find out. Would be good to try and replicate it as accurately as possible.

I'm still searching for the second part of the desk block diagram, will have to brave the horrors of my workshop to look for them... :'(
Title: Re: Hayes Middlesex Pre/Channel
Post by: hitchhiker on February 12, 2016, 09:51:42 PM
Very interesting!

In the meantime could those of us not aware of this console until now get some info?  For example:

Was this a broadcast or studio console?

What were the years it was in service?

 If it was a studio console  were there any notable recordings with it?

OK,  SILLY QUESTIONS,  I GET IT NOW!
Title: Re: Hayes Middlesex Pre/Channel
Post by: ding on February 12, 2016, 10:06:51 PM
Just google Hayes Middlesex console and the sequence of numbers on the first post. I'm guessing leadbreath is keeping it on the DL for a reason but it's not hard to figure out.
Title: Re: Hayes Middlesex Pre/Channel
Post by: hitchhiker on February 12, 2016, 10:17:27 PM
Not getting anything?  Can you post a link to what you find?
Title: Re: Hayes Middlesex Pre/Channel
Post by: Dylan W on February 12, 2016, 10:27:38 PM
Not getting anything?  Can you post a link to what you find?

Try downloading the posted files and see if that helps.  ;)
Title: Re: Hayes Middlesex Pre/Channel
Post by: ding on February 12, 2016, 10:30:35 PM
I pm'd you just in case. A channel of this would be amazing! I couldn't tell by the downloads but what kind of in and out iron for this beast?
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on February 13, 2016, 03:19:25 AM
Awesome Ian, thanks that makes more sense now.

I've also  realised that we are missing another important piece - The drawing for the Band pass filter?
I've heard that this is an important part of the sound of this desk, not sure what frequencies we're talking of here but I can try and find out. Would be good to try and replicate it as accurately as possible.

It probably uses the same topology as the other two. The treble circuit is already a peaking boost but shelving cut. It would be trivial to alter it so it was peaking for both boost and cut. I would not be surprised if the set of frequencies used followed those of the well known brilliance control.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: hitchhiker on February 13, 2016, 11:50:21 AM
Thanks Ding!  I don't know much about the that console, Hayes Middlesex was news to me!
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 13, 2016, 04:29:55 PM
Quote
  I pm'd you just in case. A channel of this would be amazing! I couldn't tell by the downloads but what kind of in and out iron for this beast?
 

Input and Output iron can be sourced from Mr Brian Sowter:

http://www.sowter.co.uk/vintage-audio-transformers.php
Title: Re: Hayes Middlesex Pre/Channel
Post by: hitchhiker on February 13, 2016, 05:35:58 PM
Were the line inputs unbalanced?
Title: Re: Hayes Middlesex Pre/Channel
Post by: ding on February 13, 2016, 07:49:49 PM
Quote
  I pm'd you just in case. A channel of this would be amazing! I couldn't tell by the downloads but what kind of in and out iron for this beast?
 

Input and Output iron can be sourced from Mr Brian Sowter:

http://www.sowter.co.uk/vintage-audio-transformers.php

I guess I meant what ratios for the in/out. Sowter is a little too pricy for me in the us but I can talk to David at cinemag and he can probably wind me up something.
Title: Re: Hayes Middlesex Pre/Channel
Post by: hitchhiker on February 14, 2016, 11:04:34 AM
Sowter page states,

input-  Ratio 1:3.16 with -5 dB, -10 dB and -15 dB secondary taps. Very large 100% Mumetal core and thick Mumetal can. Colour coded leads. Primary inductance 15H approx -0.5 dB at 8 Hz with 200 ohm microphone.

output- 100% Mumetal output transformer ratio 1:1.77. Total DCR referred to secondary 160 Ohms aprox. Max level +18dBu at 20 Hz
Title: Re: Hayes Middlesex Pre/Channel
Post by: pucho812 on February 14, 2016, 07:23:28 PM
not sure  if anyone has seen this yet?
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 15, 2016, 09:58:30 AM
Handy manual that, makes so much more sense once you have some schematics to refer to when reading.

I see they mention the Band Pass has a Cut at 25Hz and 25kHz can someone smarter than me implement this circuit using the topology of the Bass/ Treble circuit?

I was also wondering if it could be possible to implement a switch with the frequencies mentioned in the manual within the Treble circuit so it can be used in the Presence mode too??

Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 17, 2016, 05:12:06 AM
Allright looks like I'm on my own on this one

Ive got a rough idea at implementing the circuits as per previous posts problem is my simulation software is not working  as PC is knackered.
I'm busy rebuilding the PC and will update if the mods mentioned are feasible.
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on February 17, 2016, 05:45:29 AM
Allright looks like I'm on my own on this one

Apologies, I would have responded but I was rather ill yesterday. Really bad stomach ache. Doctor thinks I may have gall stones. Got to go back today and have blood tests and possibly an ultrasound.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on February 17, 2016, 08:22:43 AM
Hey Ian

No need to apologize ,  you've already been a great help on this project and countless others.

Hope you get better soon, I have also had some issues with gall bladder and its not nice.
Strangely my guts were fine but I had a really bad pain above my right shoulder blade?!!
What worked for me was artichoke extract capsules, never had any more issues since(touch wood)

Mick
Title: Re: Hayes Middlesex Pre/Channel
Post by: skeeler87 on February 20, 2016, 11:30:58 AM
Hey Guys!

What are these resistors labelled "AOTI" in the input and output amplifiers? Also does the part labelled "Gain Control" just supposed to connect to a potentiometer or is it suggesting that it leads to a different circuit? Sorry if these questions are elementary, still learning a lot of this stuff!
Title: Re: Hayes Middlesex Pre/Channel
Post by: hitchhiker on February 20, 2016, 03:52:18 PM
My guess is AOT1 says  Adjust On  Test One
Title: Re: Hayes Middlesex Pre/Channel
Post by: warpie on April 26, 2016, 02:36:41 PM
Right here we go:

Input Amp

Is this supposed to be the mic preamp section? If so, I wonder how the nesessary gain is achieved. As far as I can tell this circuit will provide a gain of around 20dB + whatever extra gain the input Tx can achieve. Am I missing something?
Title: Re: Hayes Middlesex Pre/Channel
Post by: ding on April 26, 2016, 05:16:52 PM
I imagine with the output amp.
Title: Re: Hayes Middlesex Pre/Channel
Post by: warpie on April 26, 2016, 07:03:58 PM
I imagine with the output amp.

Well, the output amp  (and an output Tx) will definitely add more gain but I can't see the reason of having such a small gain in the input amp. Unless it's not a mic preamp, I'm definitely missing something :)
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on April 27, 2016, 07:43:02 AM
I imagine with the output amp.

Well, the output amp  (and an output Tx) will definitely add more gain but I can't see the reason of having such a small gain in the input amp. Unless it's not a mic preamp, I'm definitely missing something :)

From what I remember of the manual, the internal operating level is -10dBV (where 0dBV = 0.477V into 200 ohms). The smallest input level is -50dBv so the gain needs to be 40dB. The amp has 25dB and the transformer secondary taps give up to 15dB for a total of 40dB.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: warpie on April 28, 2016, 05:35:49 AM
From what I remember of the manual, the internal operating level is -10dBV (where 0dBV = 0.477V into 200 ohms). The smallest input level is -50dBv so the gain needs to be 40dB. The amp has 25dB and the transformer secondary taps give up to 15dB for a total of 40dB.

Cheers

Ian

Thanks for chiming in IAN. If I remember well the transformer is 1:3 (or something like that) so I'm not sure how it can give 15dB of gain but I get the idea. Do you believe there's an attenuator somewhere and if so, what would be the best place to have it? If it's between the transformer and the input of the amp , it would change the impedance, right?
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on April 28, 2016, 09:53:30 AM
From what I remember of the manual, the internal operating level is -10dBV (where 0dBV = 0.477V into 200 ohms). The smallest input level is -50dBv so the gain needs to be 40dB. The amp has 25dB and the transformer secondary taps give up to 15dB for a total of 40dB.

Cheers

Ian

Thanks for chiming in IAN. If I remember well the transformer is 1:3 (or something like that) so I'm not sure how it can give 15dB of gain but I get the idea. Do you believe there's an attenuator somewhere and if so, what would be the best place to have it? If it's between the transformer and the input of the amp , it would change the impedance, right?

Check out the document in this post:

http://groupdiy.com/index.php?topic=61769.msg783662#msg783662 (http://groupdiy.com/index.php?topic=61769.msg783662#msg783662)

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: L´Andratté on April 29, 2016, 03:47:51 AM
Check out the document in this post:
The man,who document, that wasn´t there...
Apart from the dark mysteries surrounding this console, it´s cool looks and rather prominent former users, does it actually sound so special as to justify the hype (sorry for my ignorance, of course it does, but is anyone here, actually having heard it in real life)?
The circuits are very simple looking, not a bad thing at all, but evoking recollections of my early fuzz face experiments... ;D
Title: Re: Hayes Middlesex Pre/Channel
Post by: L´Andratté on April 29, 2016, 06:19:28 AM
..., does it actually sound so special...

Now, the more I look into it I really like to find out for myself.
BC109 is readily available, ACY21 not so, but 2N222 (not 2N2222), a germanium PNP that is supposed to work as equivalent, for acceptable expense.
Brian Sowter thankfully makes In-/Output transformers (as stated above), sadly at the doubtlessly justified price of 91 GBP each, which is not an impulse buy for me atm... Hmmm *Brood*
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on April 29, 2016, 07:31:53 AM
As to the sound, I think the key aspect is that it is all class A, just like the classic Neve designs of the 70s. The circuit details are not terribly relevant. That and the use of transformers are in my view the two main contributors to the sound.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on April 29, 2016, 08:56:30 AM
Its definitely not a circuit with massive amount of gain, I know of someone who has added an extra gain stage after the output amp to increase gain.
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on April 29, 2016, 09:55:56 AM
Its definitely not a circuit with massive amount of gain, I know of someone who has added an extra gain stage after the output amp to increase gain.

The basic mic pre input sensitivity seems to be -50dBu which is more than adequate for the majority of uses of the mixer. They were not trying to record small acoustic signals at large distances with ribbon mics. They mostly used condensers for which -50dBu sensitivity is plenty.

Cheer

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: leadbreath on April 30, 2016, 02:30:47 AM
Quote
The basic mic pre input sensitivity seems to be -50dBu which is more than adequate for the majority of uses of the mixer. They were not trying to record small acoustic signals at large distances with ribbon mics. They mostly used condensers for which -50dBu sensitivity is plenty.

Cheer

Ian

true...
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on April 30, 2016, 05:57:01 AM
Quote
The basic mic pre input sensitivity seems to be -50dBu which is more than adequate for the majority of uses of the mixer. They were not trying to record small acoustic signals at large distances with ribbon mics. They mostly used condensers for which -50dBu sensitivity is plenty.

Cheer

Ian

true...

OTOH, if you are designing a mixer for use in a TV studio or film set, where the mic has to be out of shot, then you might need the 80dB gain available from a Neve channel.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: Michael Tibes on April 30, 2016, 07:10:11 AM
No comp/limit  at the moment unfortunately, my source has said that he has a loads of drawings from that company but only managed to find the ones I mentioned so far in relation to the "numbers" desk.

Awesome Info!!  :) :)

So the comp different from the ..413 (which we know already)?

If I get things right then this should shed some light on the relevant history:

http://www.waves.com/behind-abbey-road-studios-emi-tg12345-consoles
http://www.musictech.net/2014/07/emi-tg12345/

Michael
Title: Re: Hayes Middlesex Pre/Channel
Post by: xazrules on March 01, 2019, 12:53:34 PM
hi, sorry if i exhume this zombie thread but i don't think is necessary to open a new one.
so, i've build this two stages with the acy21 as output and bc549c as the other four trans and an oep a262a3e wired 1:12 and i have about 50db of super flat 20hz to 20khz gain but i've omitted the output transformer. IT IS VERY NOISY too be usable as preamp, is it normal for this circuit?  is there something that i can do to improve the SNR? use the original transistors? oh, and i've used one 10k pot at the input after the transformer and one 10k pot after the output amp. beautiful shhhmooth sound anyway.

thanks!
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on March 01, 2019, 01:19:03 PM
There is no reason why this preamp should be very noisy. First check with just the first stage. You should have close to 50dB max gain. Short the transformer input and measure the noise level at the output. It should be -70dBu or better.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: JohnRoberts on March 01, 2019, 01:44:56 PM
Well, the output amp  (and an output Tx) will definitely add more gain but I can't see the reason of having such a small gain in the input amp. Unless it's not a mic preamp, I'm definitely missing something :)


From what I remember of the manual, the internal operating level is -10dBV (where 0dBV = 0.477V into 200 ohms). The smallest input level is -50dBv so the gain needs to be 40dB. The amp has 25dB and the transformer secondary taps give up to 15dB for a total of 40dB.

Cheers

Ian
That is a little weird use of dBV... 

0dBm (1 mW) is indeed 0.477V @ 200 Ohms, a power level with 0dBm=1mW.

dBu was commonly voltage wrt 0.775V without specific impedance qualification.  dBV was voltage wrt 1V again without impedance qualification...   -10dBV was (is?) a very popular semi-pro recording equipment 0VU reference level (0.316V) again independent of termination.

I recall another dBV ( x) voltage reference with stated OdB voltage reference level included inside parenthesis. For this case that could be stated X dBV(.477V). dBV without stated reference implies a 1V voltage reference.

I've seen dBv used interchangeably with dBu (voltage ratio wrt 0.775v)

Of course there are any number of variants in popular dB usage.

JR
Title: Re: Hayes Middlesex Pre/Channel
Post by: xazrules on March 01, 2019, 01:53:42 PM
There is no reason why this preamp should be very noisy. First check with just the first stage. You should have close to 50dB max gain. Short the transformer input and measure the noise level at the output. It should be -70dBu or better.

Cheers

Ian

yes Ian thanks, if i short the trx it disappears. so why all that noise? meanwhile one of the channel started motorboating, a bad cap?
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on March 02, 2019, 12:53:00 PM
yes Ian thanks, if i short the trx it disappears. so why all that noise? meanwhile one of the channel started motorboating, a bad cap?

It is just picking up stuff in the air. Mic pres do that if there is nothing connected to their inputs. They are pretty sensitive.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: xazrules on March 03, 2019, 07:02:44 AM
yes i see, changed the steup ratio of the transformer improved things, but are there some techniques to make the amp see a costant load? i ask this because if i want to use only the line in (summed to the input beetwen trx and amp with a series resistor) with no mic connected the SNR worsen..
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on March 03, 2019, 03:44:33 PM
yes i see, changed the steup ratio of the transformer improved things, but are there some techniques to make the amp see a costant load? i ask this because if i want to use only the line in (summed to the input beetwen trx and amp with a series resistor) with no mic connected the SNR worsen..

If you changed the step up ratio of the transformer downwards then the noise would go down simply because the gain has gone down.

I don't understand your method for including a line input. Can you post a schematic showing how you do it.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: xazrules on March 04, 2019, 03:17:59 AM
yes this is how i summed mic and line togheter. if both the inputs are connected the noise performance is perfect, but if i remove a mic from his input it worsen.
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on March 04, 2019, 03:58:03 AM
OK, now I understand. I have not seen a line input summed like this before. I think the problem is it is not really summing because the impedance appearing across the transformer secondary depends on what is plugged into it. With a mic plugged in this impedance is the mic impedance reflected to the secondary. This appears in parallel with the 10k pot and so attenuates the line input signal and the overall noise. With no mic plugged in the reflected impedance in parallel with the 10K pot is a lot higher so there is less attenuation so the noise appears to be higher.

The normal way to this would to to use a switch to connect the 10K pot wither to the line input of the mic transformers secondary.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: xazrules on March 04, 2019, 04:50:18 AM
thank you for your reply it make sense, and yes you've not seen something like this because this time i feel creative and i've tried to be very very cheap, a line/mic switch will work for sure. but can't we shunt the primaries of the trafo with a resistor to change its ratio behaviour? or am i going too hippy thinking?  :D
Title: Re: Hayes Middlesex Pre/Channel
Post by: ruffrecords on March 04, 2019, 09:10:21 AM
thank you for your reply it make sense, and yes you've not seen something like this because this time i feel creative and i've tried to be very very cheap, a line/mic switch will work for sure. but can't we shunt the primaries of the trafo with a resistor to change its ratio behaviour? or am i going too hippy thinking?  :D

The 'normal 'way to combine a mic and line input is to do it on the primary side of the transformer. So add a 1K2 resistor across the primary winding. Add a series 10K resistor from each side of the primary winding to the hot and cold inputs of your line input. The mic input is connected right across the 1K2.

Cheers

Ian
Title: Re: Hayes Middlesex Pre/Channel
Post by: xazrules on March 04, 2019, 10:57:19 AM
thank you, i'm going to use this solution for sure. i was thinking that a transformer could isolate the two different input from each other. i will never understand transformers!
Title: Re: Hayes Middlesex Pre/Channel
Post by: waltzingbear on July 26, 2019, 01:39:17 PM
Those schematics here are lovely and give a good insight into the design of this classic console. But we don't have the schematics for Amp B or C, which combined describe the summing characteristics of this desk, which in my experience is unique.

Anyone have them to publish here? Most of it can be derived from the description in the pdf Mk II manual, but it would be nice to have the schematics in full.

Cheers
Alan