Opamp Labs EQ questions

Hey guys,

I'm building this EQ:

Full size pic is here:
http://www.angelfire.com/ky3/craigorris74/opamplabs-109eq.jpg
Got a question about the schematic, though. I've never seen the ground with the dashed circles attached to the circuit before. Should I ground these points?

Thanks,

Craig

I hope your kidding.... Just in case you're not, the dashed line represents a 3-pole EQ In/Out switch and the little circles represent the terminals on the switch. Not unlike the dashed line on the filter I/O switch.

--Vince

Not what I'm talking about. See several of the ground symbols around the amp-they're tied to the circuit with dashed circles. I've never seen that before. I know about the switches.

They indicate cable shields or shielding tracks on a PCB.

Great, thanks for the tip.

Craig

[quote author="Vince@Speck"]I hope your kidding.... [/quote]
meaning....?

i'm not sure we're all talking about the same thing... if you mean the dashed horizontal line that connects the three switches marked SW1A, B, and C then that is one 3PDT switch, the EQ in/out switch. when the switch is "in" all three places are connected to ground and the filter network does its thing. when in the "out" position, all three places are hanging in space and there is no AC path to ground through the filters, so the EQ does nothing, but signal still passes through the opamp.

the dashed lines do not represent traces or shields, all three of those places are really located together in one switch. the dashed lines indicate they all switch together because of this.

ed

[quote author="edanderson"]i'm not sure we're all talking about the same thing... if you mean the dashed horizontal line that connects the three switches marked SW1A, B, and C then that is one 3PDT switch, the EQ in/out switch. when the switch is "in" all three places are connected to ground and the filter network does its thing. when in the "out" position, all three places are hanging in space and there is no AC path to ground through the filters, so the EQ does nothing, but signal still passes through the opamp.

the dashed lines do not represent traces or shields, all three of those places are really located together in one switch. the dashed lines indicate they all switch together because of this.

ed[/quote]

Have a look at the circuit posted. Of course he doesn't mean the dashed lines.

He is refering to the little dashed circles with grounds attached. These refer to either shields or guard rings, used to shield PCB traces.

OK, where did you get the original inductors? Does opamp labs still makes it? Or if not the original you are using, what are the values? You would need big calculations to find out, or opamplabs to tell it... I would love to know. I do have myself some 15-20 of the 325 opamps.

Does anyone have a circuit for the 325?

Looks interesting.

Peter

Rafa:

http://www.opamplabs.com/87.htm

Peter:

I believe the 325 is essentially a 425 opamp with some internal resistors added. If I recall correctly, Bel Losmandy wrote words to this effect in an AES paper on a portable mic mixer he'd designed.

See below for a 425 opamp used in an EQ application, then compare to circuits using the 325.



As for getting the specifics of the 425 circuit, good luck. There are no schematics out there that I'm aware of. Someone who does know the circuit has stated that it's a two-transistor hybrid input stage followed by a discrete output stage.

I've been collecting 325s with the intention of building some 109 EQs but I just never seem to get around to it. I also never got around to inquiring with Opamp Labs as to the availability of the inductors.

[quote author="gswan"]
Have a look at the circuit posted. Of course he doesn't mean the dashed lines. [/quote]

sorry to have overlooked the grounded shields on the right hand side. i did not see them. you are correct. craig's description didn't match up with what i first saw, which is why i started my comment with...

[quote author="edanderson"]
i'm not sure we're all talking about the same thing...
[/quote]

thanks for pointing out my error and identifying the shields.

since there are colors given next to most of the traces with the grounded shield symbols attached, i would suspect that these connections were made with shielded colored wires, with the shields tied at one end. this would make sense since the switches in the opamp labs modules i've seen are not PCB mount.

craig, good luck with your build, and sorry if my post added to the confusion. to rafa, the inductors are still available as NOS from opamp labs last time i checked.

ed

Thanks Dave!

mmm... $60.00 for each set of inductors needed for one channel...

You can get all the plugin modules and inductors from opamp labs, and I've bought big lots of the stuff cheap on Ebay.

[quote author="NewYorkDave"]
Someone who does know the circuit has stated that it's a two-transistor hybrid input stage followed by a discrete output stage.
[/quote]
S.D.?

Yep, SD and his X-Ray.

and I've bought big lots of the stuff cheap on Ebay

Click to expand...

Speaking of which:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=230172563056&ssPageName=ADME:B:SS:US:1

Peter

Hey if anyone is interested I have a drawer of 325s and 425s. I also have a 10 channel mixer with EQ, not sure I want to part with that yet. I don;t need all the modules I've got though. I'd have to look to see how many I've got, but it's quite a few. Let me know if you are interested and we can work out a price.

Michael Stucker
Wolfgang Michael Sound