spectrasonics EQ info

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

pablobolche

Well-known member
Joined
May 21, 2009
Messages
98
Location
CHILE
Hello ..
I received a very poorly racked and not working spectrasonics 101 pre + eq from a friend who asked me to get it up and running...

I am searching info on this EQ(foto attached..) no faceplate..

it is passive inductor based..
has 4 switched pots...hooked it up passive after a 600 ohm trafo and trying it now ..
2 for freq selection and 2 for gain...and one affects highs and the other lows..but can't really tell as there is more bass turning pots CCW and highs seem to  behave the same way.. might be a HPF LPF??

Freq selection switches when fully CCW seems to bypass the EQ...


All if is welcome...



 

Attachments

  • IMG_20170803_133722.jpg
    IMG_20170803_133722.jpg
    1.8 MB · Views: 95
if it is genuine Spectra, it is not a 600 ohms passive unit.
it goes in the feedback loop of companion amplifier, similar to UA 508.
contact Mutterd, he knows them well.
 
yes this came with 101 cards  and when I revised these connections I saw that the EQ in/out conections with the card are different..
the EQ in comes from 101 out...which is THE OUTPUT and EQ out goes to the Feedback in??
this  was my following question...

Thank you...
 

Attachments

  • IMG_20170803_145228.jpg
    IMG_20170803_145228.jpg
    1.7 MB · Views: 57
for more info you should contact Timothy aka mutterd

https://groupdiy.com/index.php?topic=56300.msg789876#msg789876

http://www.spectra-sonics.com/

 
Ok - so a few things first off - that is indeed a Spectra Sonics model 500EQ - and yes, the EQ is passive and is made to be used in the feedback loop of a Spectra 101 or 110 - it can technically be used passively, with make up gain, but all the controls will be backwards...

you're right - there are 8 frequencies total split between high and low and there is an off position on each  - frequencies are 50Hz, 100Hz, 200Hz, 300Hz, for the low and 2.5kHz, 5kHz, 10kHz and 15kHz for the high - boost and cut is +/- 12dB in 2dB steps.

also - that 101 card you have has been tampered with and chances are about 100% that it wont meet spec as is - from the looks of it, at the very least the output transistors have been swapped and the cards are not the kind of thing where you can just arbitrarily swap components - save yourself the headache and get them fixed.

as for hooking them up - there are 2 ways - and it depends how the amplifier its engaged in is terminated:

If the amp is terminated resistively - as in driving a fader or another 101/110 stage - the EQ hangs between the output (+) and ground and references the feedback point - so that is EQ A to 101 Pin H, EQ B to 101 Pin F and EQ C to 101 Pin L respectively.

If the amp is to drive an inductive load like a transformer then the EQ sits between the DC Signal Output point thru a 10uF 35VDC Tantalum and ground and also references the feedback point - so EQ A to Pin K thru a 10mF 35VDC Tantalum, EQ B to Pin F and EQ C to Pin L respectively.

beyond that, I should be able to provide any info you might need, but I'm going to need some more info on how you want to hook everything  up - any way you could give me a run down of the design parameters?
Input/Output Transformers?
how much gain total per channel?

hope this helps - looking forward to hearing more about the project.
Timothy



 
Hi Timothy

I have 2  101 cards and 2 500 eq's....

was thinking of using vigortronix 1+1 : 2+2 / 150 + 150 : 600 + 600 inputs trafos as to have a switchable 150ohm-600ohm input (and "I have these in my trafo box")

will add pad, polarity and 48v switches....

As the EQ uses the feedback loop.......where would I put a gain pot?? Switched pot??

regarding output my friend wants balanced output...but also some kinda Fader/output volume control
I have some DRV134 and some 1:1 output trafos...but don't really know which way to go...

mutterd said:
it can technically be used passively, with make up gain, but all the controls will be backwards...
Great to know as I was getting all confused with how the controls operated..

I have a live gig tomorrow so can't get into this till monday...

Thank you all for all your comments








 

Attachments

  • Captura de pantalla 2017-08-03 a las 22.15.34.png
    Captura de pantalla 2017-08-03 a las 22.15.34.png
    1.2 MB · Views: 36
Hey Pablo - see below for some thoughts.

[quote author=pablobolche]
Hi Timothy

I have 2  101 cards and 2 500 eq's....

was thinking of using vigortronix 1+1 : 2+2 / 150 + 150 : 600 + 600 inputs trafos as to have a switchable 150ohm-600ohm input (and "I have these in my trafo box")

[/quote]

I had never heard of these before but just checked out the specs - i like the form factor but i don't think you should go 150:600 as the input impedance of the 101 cards is 604Ω - with a step up input you'll be reflecting only 150Ω to the mic which might be ok for some dynamics but most condensers and ribbons really wont like this. I would suggest using the 600:600 model, the VTX-101-006 if you want to use that brand, else there are many modern and vintage options available - Spectra traditionally used the Triad a67j's.

[quote author=pablobolche]
will add pad, polarity and 48v switches....
[/quote]

yup - and  id suggest some sort of switchable input pads - Spectra used -10/-20/-30dB pads on their consoles - I have had good luck with that or -6dB steps on a rotary switch as well. I have designed an input PCB for a console build that accomplishes all of this - some other members have expressed interest in these - maybe i'll try to make a batch later this month - that would be a simple solution for you...


[quote author=pablobolche]
As the EQ uses the feedback loop.......where would I put a gain pot?? Switched pot??
[/quote]

you wouldn't - Spectra is, by design, a fixed gain amp and when the EQ is utilized in the feedback loop it sets the cards for 40dB of gain - which is the best option in terms of input sensitivity and noise response.

[quote author=pablobolche]
regarding output my friend wants balanced output...but also some kinda Fader/output volume control
I have some DRV134 and some 1:1 output trafos...but don't really know which way to go...
[/quote]

I have no experience with the DRV134's but id suggest using an output transformer - another 600:600 - Spectra used the Triad HS66 but there are countless contemporary and vintage options - in which case you can keep it single ended out of the 101 and drive the output transformer from an opamp of your choosing. The other (better) option is using 2 101's per channel and keeping it all discrete. (Either way) You can add your fader/output level control between the first 101 and the output amp - which buffers the attenuator and allows you to drive the output transformer from a fixed low impedance which will give you better results from the transformer.

In this case you'd hook the EQ up the first way i explained (between output (+) and ground.)

[quote author=pablobolche]
Great to know as I was getting all confused with how the controls operated..

I have a live gig tomorrow so can't get into this till monday...

Thank you all for all your comments
[/quote]

next we'll have to talk grounding, witch is really the biggest thing with these spectra amps - more than likely it was improper grounding which took out the output transistors on your cards - improper grounding leads to oscillations and/or unchecked RF burning away at the amp - it will wreck your noise specs, kill your headroom and and you'd never know till it was too late - the 101 and 110 will indiscriminately amplify RF or high frequency oscillation just the same as if it were your kick drum mic... If you read back on the forums to other people attempting to rack up spectra there are all sorts of wacky things that have been suggested - these are all various levels of kludge to try to overcome poor grounding techniques...

have a good gig and we can talk more next week.








[/quote]
 
Hi Timothy
mutterd said:
I would suggest using the 600:600 model

would a dynamic mic be happy to see a 600Ω input impedance?? should I have a 150Ω input option for dynamic mics?

I have some more questions but need to get the input ok before I get into the guts...

thanks












 

Attachments

  • Captura de pantalla 2017-08-10 a las 10.43.02.png
    Captura de pantalla 2017-08-10 a las 10.43.02.png
    36.8 KB · Views: 35
Hey sorry - I missed this last post.

pablobolche said:
would a dynamic mic be happy to see a 600Ω input impedance?? should I have a 150Ω input option for dynamic mics?

the 604Ω would be the minimum load seen by the mics - and from my experience, yes - dynamic mics are absolutely fine with it - so id say the 150/600 switch is not necessary.

but the reality is I never really use the mic pre's without needing at least one stage of the pads engaged so your input impedance can be a adjusted by design of the pads. just remember that the series legs of the pads are going to contribute to the overall noise level so there is an advantage to keeping them as low impedance as possible.

even tho the 150Ω or 200Ω spec of the microphone is frequency dependent... and i have NEVER experienced any signal degradation with the 604Ω input, I usually design the pads to be closer to 1kΩ reflected to the mic cause all the original Neumann documentation states that the mic should be terminated with 5 times its output impedance for proper transfer...

in regards to your drawing:

1. as i said - don't see any point in the 150 to 600 ohm switch for the primary - plus as drawn its going to pop if actuated when phantom is engaged and also is going to change the values of the pad depending on position...

2. I would put the pad and phase reverse after the transformer else there will be a click/pop if you switch either the pad or phase with phantom engaged which will be amplified by the total gain of the circuit. Also, with the pads on the secondary side of the transformer they can be unbalanced - on the primary, unless the resistors are matched you are messing with the CMRR of the TX ...

3. what are CC18/CC19 for?

finally - remember that you have a fixed gain amp stage so i would design at least a 2 step pad (-15/-30) but preferably a 3 step (-10/-20/-30)...

hope this helps.
Timothy


 
Hi Timothy ...

mutterd said:
the 604Ω would be the minimum load seen by the mics - and from my experience, yes - dynamic mics are absolutely fine with it - so id say the 150/600 switch is not necessary.
OK ..no 150/600 switch

mutterd said:
2. I would put the pad and phase reverse after the transformer else there will be a click/pop if you switch either the pad or phase with phantom engaged which will be amplified by the total gain of the circuit. Also, with the pads on the secondary side of the transformer they can be unbalanced - on the primary, unless the resistors are matched you are messing with the CMRR of the TX ...
OK .. I have attached a new schematic...
using a unbalanced L Pad 12 position switch adjusted for 1k shunt and going from 0 to -44db in 4db steps
this gives me a series resistor maximum value of 157k...would this be reasonable noisewise??
Gain on the SS101 could be set to 44db (R Feedback 20k) so fully CCW would be unity gain..

mutterd said:
3. what are CC18/CC19 for?
these comes from my an api type input schematic I use.. I have deleted them..


mutterd said:
The other (better) option is using 2 101's per channel and keeping it all discrete. (Either way) You can add your fader/output level control between the first 101 and the output amp - which buffers the attenuator and allows you to drive the output transformer from a fixed low impedance which will give you better results from the transformer.
I bought some NOS Fairchild 2n3566 so I might end up making a couple of DIY SS101 so as to do this better option..

thank for sharing your knowledge..






 

Attachments

  • Captura de pantalla 2017-08-23 a las 23.13.02.png
    Captura de pantalla 2017-08-23 a las 23.13.02.png
    49.1 KB · Views: 44
Hey
While we're talking spectra 101s, I've got a question that's slightly off the OPs topic...

Does anyone know the max gain that can be set via feedback resistor?  I know 50db with 42k r is possible.  Can 55db be obtained with an 84k r? Or even 605th with 164k r?  At what point do things become unstable?
 
enginefire said:
Hey
While we're talking spectra 101s, I've got a question that's slightly off the OPs topic...

Does anyone know the max gain that can be set via feedback resistor?  I know 50db with 42k r is possible.  Can 55db be obtained with an 84k r? Or even 605th with 164k r?  At what point do things become unstable?
You forgot:
"At what point do things become unstable noisy ?"

With a feedback resistor of 10k most of the common op-amps (e.g. NE5532) outperform the resistor generated noise.  So with 164k stability will not be the only problem.....


 
analogguru said:
You forgot:
"At what point do things become unstable noisy ?"

With a feedback resistor of 10k most of the common op-amps (e.g. NE5532) outperform the resistor generated noise.  So with 164k stability will not be the only problem.....

I don't think this spectra card behaves like an op amp... Could be wrong but it's dead quiet using a 42k r (as per it's manual at that).
 

Latest posts

Back
Top