Hayes Middlesex Pre/Channel

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leadbreath

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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???
 

leadbreath

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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!!
 

ruffrecords

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leadbreath said:
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
 

leadbreath

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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.
 

ding

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leadbreath said:
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?
 

leadbreath

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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.
 

leadbreath

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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
 

ruffrecords

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leadbreath said:
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.
 

leadbreath

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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... :'(
 

hitchhiker

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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!
 

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