Feeler: Quad 8 444 EQ with M/S extension in 51x format.

[dmnieto]

Hi all,

I am working in doing a 51x design for a mastering EQ based on the Quad-8 444x. I had the opportunity to listen to that EQ a while ago, and I loved its sound.
It is a four-band with selectable HP/LP filters.
It is going to be designed by mono units with a header connection to build the stereo M/S matrix.

I am trying to make it transformer in/out as per the original design with input QEE 3436 and output QEE transformers 3041. It is going to have some tricky items like the fact that for everything to fit the frequency/boost-cut selectors will need to be in a single concentrical unit (I have managed to put together something similar by assembling three lorlins together :S)

Also, the original had a +/- 12 dB on 1-2dB steps. I am probably thinking on reducing that to +/-6dB with 0.5/1dB steps. Or maybe using center detent (but that will make matching difficult)

so what do you think?
https://soundcloud.com/david-martinez-nieto/sets/ams-q444-prototype-sound-samples


Components:

pcbs: ($25)
dual switches: http://www.digikey.com/short/fdqmd ($82.02)
main components: http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=2395388f79 ($140.66)
Output transformer: QEE-3041 $23.54 [From Cinemag]/ Edcor XSM600-600 $12.97 https://www.edcorusa.com/xsm600-600 / JT-11SS-DLCF http://www.jensen-transformers.com/prices.html#11ssdlcf
Input transformer: QEE-3436 $53.55 [From Cinemag]/ Edcor XSM10K-600 $12.97 https://www.edcorusa.com/xsm10k-600
AM10: http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=9da5df04fd ($11.50)
DC-DC: http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=c748c4aa51 ($16.21)
Metal frontplate: $50
Knobs: http://classicapi.com/catalog/product_info.php?cPath=23_40&products_id=362 (9x4.76 $42.84)
Total price per module: $393.23 (cinemags) / 343.23 (Edcor)


I have updated the first thread to track interest on a group buy. Also if you want me to try to get opamp boards, transformers, switches etc

Name​

Boards

Panel

transformers

Switches

goetzmd	2	Yes	Yes	Yes
dmnieto	2	Metal	Yes	Yes
Upacesky	2	N/A	N/A	N/A
kvothe	2	N/A	N/A	N/A
Indecline	2	N/A	N/A	N/A
rob_gould	2	N/A	N/A	N/A

 

[dmnieto]

These are the renders of the two opamp options for the make-up gain that I have... Both have been done in API2520 format in case anybody wants to try different flavours.

The top row (AM10B) is the circuit as in the 444x schematics, on the bottom row it is the Quad AM-10 that is also used in some of the Quad 8 consoles.

 

[Upacesky]

Well I might be interested, but aren't those transformers are quite hard to get? And it seems that center detent is not the best for matching (at least not for a mastering grade unit)

 

[Baltimore]

I'm very interested in this, and also that am10 in 2520 footprint. I'd personally be more into the full 12db cut/boost, rather than the proposed 6. In/out transformers are cool but yeah, the originals would be near impossible to find I think. Footprints for the usual suspects (cinemag/jensen/Ed Anderson/edcor) might make things a bit easier.

 

[emrr]

Someone else here once knocked up the AM-10 in a 2520 footprint, and I've never again spotted it. 

 

[dmnieto]

I was checking with grayhill, and the particular switches we needed are about $43 the piece for a minimum order of 25. (http://www.sager.com/71cf30-1bz-2bz-3440651.html) So I have worked out an alternative with a pair of non concentric switches... It is a very tight fit, but it should work (I have started the front panel design too).

For the transformers, I have contacted with David at Cinemag which has the transformers... let me copy his answer:

All of the QuadEight transformers were engineered and built by our Reichenbach Engineering division. 

The QEE 3436 design was last updated on January 10, 1974.  It was 5K/20K:600, and was made on the same core size as the CM-2810.  It used 50% nickel alloy for the laminations.  We only stock high-nickel, which is 80%.  Mixed 50/50 with steel, it sounds a lot better.  The 50% material can be obtained, but there is a significant minimum run and, again, it does not sound as good.

The QEE 3041 used the same core size but with 29 M6 steel.  It was a bifilar transformer, but with more turns than the CMOB-2S with smaller diameter wire.  ("S" means steel laminations.)  It will have lower distortion, trading that for less bandwidth.

I am trying to get a quote from him, and also source some NOS from orphanaudio (~$45 a piece I think), I am considering to add in the circuit an option for electrically balancing and de-balancing for the people that want to go a bit cheaper, or want to try both options.

For the AM10's I am going to order and test... but I am going to try to design another PCB without SMD parts. I personally love them, I have built a custom 990 opamp using http://oshpark.com/shared_projects/jmPA7GAO that works incredible and it is very neat, but I inderstand the fear of some people for SMD, even if they are actually easier to solder in some cases (with the right technique they just slot in)

 

[dmnieto]

This is the front panel I am working with... the lack of concentrical switches for a good price is really hurting. But I managed to squeeze everything together by using grayhill's double deck 71s and single deck 51's on the frequency select and boost. And using subminiature spdt switches that will control the relays. I need to do a cut-out in the PCB to fit the filter switches...
unfortunatelly there is just no way to fit a pot for the gain switch, so I will use a 100K single turn trim resistor in series with a 3.16K for the gain switch , that should give a 0dB to -15dB attenuation on the input (the input transformer drops 10 dB).

the cut/boot cab be changed back to +/- 12dB with just changing the resistors...

 

[dmnieto]

Another point which is interesting is the power supply... the opamps are specified for +/- 28V rails, giving it a massive +28dBu of headroom on a 600 Ohm load. It is possible to run it on the standard 24V of the format, but it will be a pity to lose that...
So I am going to build a small add-on board to put a step-up switching regulator (I know, some people hate it and with a reason) off of the 16V, rails... I will directly run the +/- 15V from the +/- 16V as it should be safe.

 

[emrr]

to quote Eisen Audio:

When you convert power supply voltage (DC peak to peak) to theoretical dBu limits (defined as max output level if circuit is capable of rail to rail swing, which most aren't, and without step down/step up transformers on the I/O), the results are as follows:

9V (or +/-4.5V) = +12.27 dBu
12V (or +/-6V) = +14.77 dBu
24V (or +/-12V) = +20.79 dBu
30V (or +/-15V) = +22.73 dBu
32V (or +/-16V) = +23.29 dBu
34V (or +/-17V) = +23.82 dBu
36V (or +/-18V) = +24.31 dBu
40V (or +/-20V) = +25.23 dBu
44V (or +/-22V) = +26.06 dBu
48V (or +/-24V) = +26.81 dBu
60V (or +/-30V) = +28.75 dBu

Very little difference in headroom.  A doubling of voltage buys you 6dB, we are looking at about 17% increase going 24 to 28. 

 

[dmnieto]

to quote Eisen Audio:

When you convert power supply voltage (DC peak to peak) to theoretical dBu limits (defined as max output level if circuit is capable of rail to rail swing, which most aren't, and without step down/step up transformers on the I/O), the results are as follows:

9V (or +/-4.5V) = +12.27 dBu
12V (or +/-6V) = +14.77 dBu
24V (or +/-12V) = +20.79 dBu
30V (or +/-15V) = +22.73 dBu
32V (or +/-16V) = +23.29 dBu
34V (or +/-17V) = +23.82 dBu
36V (or +/-18V) = +24.31 dBu
40V (or +/-20V) = +25.23 dBu
44V (or +/-22V) = +26.06 dBu
48V (or +/-24V) = +26.81 dBu
60V (or +/-30V) = +28.75 dBu

Very little difference in headroom.  A doubling of voltage buys you 6dB, we are looking at about 17% increase going 24 to 28.

That is rail-to-rail, which the AM-10 is not. I may need to tweak the biasing of the opamp to make it work nicely with the 28V. In any case, I want to give the VPR only guys a chance to use this, so I think the power supply would be a good idea.

 

[emrr]

Regardless of rail to rail or not, I still think the basic Vx2=6dB rule applies pretty closely.  Show me the test data that proves the extra voltage buys you anything appreciable, and of practical use.  I racked a bunch of Quad Eight stuff and played with the headroom question years ago, I saw no practical difference that affected headroom standards in any meaningful way in the vast majority of cases.  If the entire chain really runs with a need for +30 headroom, you may have an argument.  My 2 cents.  It should work fine either way, so that part would be very low on my developmental list. 

 

[dmnieto]

Regardless of rail to rail or not, I still think the basic Vx2=6dB rule applies pretty closely.  Show me the test data that proves the extra voltage buys you anything appreciable, and of practical use.  I racked a bunch of Quad Eight stuff and played with the headroom question years ago, I saw no practical difference that affected headroom standards in any meaningful way in the vast majority of cases.  If the entire chain really runs with a need for +30 headroom, you may have an argument.  My 2 cents.  It should work fine either way, so that part would be very low on my developmental list.

That I agree with you, there should not be any great difference between 24 and 28 although it may be needed a slight change in biasing. What I am concerned is the VPR users... I think for those the step up will be a nice addition, I don't know how well the AM10 would work at those voltages

 

[emrr]

Gotcha.  Carry on.  I haven't hear this EQ, it looks very promising. 

 

[shabtek]

dc-dc  brings a new set of problems

nice work, cool idea.

 

[dmnieto]

dc-dc  brings a new set of problems

nice work, cool idea.

Yes I know... but I still want to find a way to allow it to run on VPR only systems

 

[ruairioflaherty]

Can you post the schematic for the original?  I'd like to see how they implement the filters.

 

[dmnieto]

Can you post the schematic for the original?  I'd like to see how they implement the filters.

I got it from here

http://groupdiy.com/index.php?topic=45258.msg698474#msg698474

Do not mind the top row of switches, there are for the read bus.

When I get this closer to completion and if there is more people interested at least 7, we can try to either do for
The concentric switches: $110 for all four ( I know, still expensive)

Non-concentric: $180 for all four...

To make the price interesting on the concentric we would a huge order, so probably the only way would be to do a group buy if there is interest in concentric switches for other projects... (but still we will be still at $142 for just the switches)

 

[dmnieto]

Ok I have finalized the schematic... Is there any suggestions before I start laying out the circuit?

 

[dmnieto]

Ok, this is the final design of the front panel before I start the routing... I am actually quite excited that I managed to squeeze everything (including a 9mm panasonic potentiometer for the input gain switch), with the non concentric switches. BTW it is following what my wife asked me in terms of functionality.

About QEE transformers, I am still waiting to hear from David from Cinemag... has anybody a suggestion on which other transformers it may be interesting to add footprint for?

The input is a 1:6 or a 1:4 step-down with {~20K/5K:600 } with up to +28dBu, the output can be either a 2:1 or 1:1, but if mounting the +/-28V with the AM10 it needs to be able to get up to +28dBu as well.

I know it is a bit early to ask who is interested, but it will be helpful in case I need to source parts (switches, transformers, pcbs and panels come to my mind).

I have not stated it clearly, but I won't be charging *anything* for this. If it works, we will do a group buy to get the lowest cost, and then I will upload the gerbers, fpd files and  eagle project for public use. I just don't want to go broke, so I will probably cap it to small order that I can manage with my own finances.

 

[goetzmd]

I would take 2 units/boards/kits…