Gyraf Calreq EQ Power Transformer

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Mylithra

Well-known member
Joined
Feb 12, 2008
Messages
184
Location
So. Texas
Im using a power supply based on the LM337/LM317 regulators and had a couple of questions.

Being that the max of both these regulators (and the 7818/7918 for Gyraf's) are both rated at a Max of 1A, would that max the max draw off the transformer a little over 2A?

would 1 of each be enough to regulate the power for 2 units?

what is the current draw per EQ?

I was planning on using something like:
http://mouser.com/Search/ProductDetail.aspx?qs=LnCj936TGbXc2loJDwYPYw%3d%3d
as my power transformer. Will this be enough?
 
[quote author="Mylithra"]Im using a power supply based on the LM337/LM317 regulators and had a couple of questions.

Being that the max of both these regulators (and the 7818/7918 for Gyraf's) are both rated at a Max of 1A, would that max the max draw off the transformer a little over 2A?

would 1 of each be enough to regulate the power for 2 units?

what is the current draw per EQ?

I was planning on using something like:
http://mouser.com/Search/ProductDetail.aspx?qs=LnCj936TGbXc2loJDwYPYw%3d%3d
as my power transformer. Will this be enough?[/quote]
Your selected transformer is a center tapped 20V. This is 10V-0-10V. Most probably only enough if you want your rail voltages at +/-8V to +/-9V.
For usual, depending on type of op-amp, you will run your circuit at +/-15VDC up to +/-18VDC rails.

As a rough estimate of 10-12.5mA per op-amp (just count the triangles) you will need beef for about 0.2A, this is 0.1A per rail for one Calrec EQ. Having a stereo unit, double this and add a little margin for LEDs or other gimmicks, so you'll need a supply delivering about 0.25A per rail.

For your required transformer voltages calculate backwards. Your LM317/LM337 will need at input min. 3V more than your desired rail voltage, so for +/-15V rail voltages you want at least +/-18V raw DC in front of your regulator. You have a silicone bridge rectifier or 4 single diodes 1N400x in front, dropping 2* ~ 0.6V. Divide this raw DC by 1.414 to get your AC voltage. Your line voltage might be 10% low, so for a 15V DC rail you need ( (15+3+2*0.6)/1.414*1.1 ) =14.9V AC secondary voltage per rail. Closest standard transformer winding will be 2*15V.
Look out for a transformer with 2*15VAC or a 30VAC ct secondary that can deliver at min. 0.5A or 2*0.25A for separate windings. Some manufacturers only quote the VA rating, this is (30VAC * 0.5A) 15VA for the example above.
(From Jakob's site he quoted a 30VA sufficient for 4 Calrec EQs, so my rough estimate seems to be not that far off)

regards
-Harpo
 
> estimate of 10-12.5mA per op-amp (just count the triangles) you will need beef for about 0.2A, this is 0.1A per rail...

No; current (mostly) flows rail-to-rail so (accepting your 10-12.5mA/opamp number) you need 19*12.5ma= >200mA= 0.2A both rails.

15V+15V at 0.2A is 6 Watts per EQ.

The power supply won't be 50% efficient, so the power transformer must be 12VA per EQ.

Two EQs want >24VA transformer, the 30VA job pointed-to is ample.

Gyraf says you can put four EQs on one 30VA transformer. I figure 48VA using "10-12.5mA/opamp". However the suggested LM833 claims 2.5mA/channel at idle. It will be higher under load. But very likely under 3 Watts total per EQ, 7VA of PT would be ample, and then 30VA carries four EQs.

However I think the "15-20Vac" should be each side of CT. Hammond would say "30VCT", and their parts would be 183J28 or 183J36.
 
[quote author="PRR"]> estimate of 10-12.5mA per op-amp (just count the triangles) you will need beef for about 0.2A, this is 0.1A per rail...

No; current (mostly) flows rail-to-rail so (accepting your 10-12.5mA/opamp number) you need 19*12.5ma= >200mA= 0.2A both rails.[/quote]
Thanks for correcting this.
 
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