Is it possible to modify 2520 DOA's for +/-24VDC?

Help Support GroupDIY:

JW

Well-known member
Joined
Jun 8, 2005
Messages
894
Location
Portland USA
Just curios if it's possible. Many of the DIY DOA's out there, it's a resistor swap and they can run at a higher voltage. Is this possible with the 2520 or no?
 

Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
2,684
Location
New Jersey, USA
Doubtful. Not without swapping a bunch of parts probably. You have to consider (at least) that some of the transistors could be subjected to 48V at startup / shutdown or high / low excursion. A lot of transistors are like 30V so pushing that to 40+V is probably too much for them. Another problem is that, unlike just about every production op amp, the 2520 has no voltage reference to set the current and just swapping a resistor might not be enough. Too much current and at the very least it will run hot. Maybe too hot.
 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
1,983
Location
Finland
I had a hunch that might be a / the reason, but that only brings up yet another "why?"-question.

The circuit clips too early? Pad down the input.
Not enough output transformer "(over)drive"? Use a smaller transformer.

But it would be nice to hear from the OP on this...
 

Harpo

Well-known member
Joined
Oct 29, 2005
Messages
2,405
Location
Rahden, Germany
Increased headroom for whatever reason might damage the next/device following piece of gear that might not have designed in safety belts in order to protect from input voltages exceeding their supply voltages. Just read the Abs.max.ratings in datasheets of not that unusual suspects, such as NE5532, NE5534, AD797, ... , stating "The magnitude of the input voltage must never exceed the magnitude of the supply voltage".
Let's hear from the OP.
 

pucho812

Well-known member
Joined
Oct 4, 2004
Messages
12,529
Location
third stone from the sun
Increased headroom for whatever reason might damage the next/device following piece of gear that might not have designed in safety belts in order to protect from input voltages exceeding their supply voltages. Just read the Abs.max.ratings in datasheets of not that unusual suspects, such as NE5532, NE5534, AD797, ... , stating "The magnitude of the input voltage must never exceed the magnitude of the supply voltage".
Let's hear from the OP.
I have never had an issue with one box having increased voltage rails feeding another box without.
For example, my avalon solid state stuff runs off bipolar 32vdc. When I feed that into a desk like an api or the like it never causes an issue. I just make sure it is not distorting. As always your headroom Max will be the lowest link in the signal chain so when you go into or from higher headroom stuff you just watch the levels and adjust so it doesn’t distort. Never seen it kill an opamp.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,157
Location
Hickory, MS
More money, older whisky, faster horses (cars), high voltage op amps... :unsure:

More signal swing is better until it isn't. Back last century Peavey service asked me to design a simple resistor pad, to scrape some output voltage from the AMR/Peavey VMP-1 tube mic preamp, because so many customers who also owned Mackie 8-bus mixers were creaming their console inputs with the hot output from the VMP-1. 😭

In general well designed gear will not be damaged by hot inputs but it is not a very practical pursuit. ☯️

JR
 

john12ax7

Well-known member
Joined
Oct 14, 2010
Messages
1,946
Location
California, US
Higher rails are not just for headroom. Sometimes things will perform better, e.g. a 12V signal might be of a higher fidelity with 24V rails instead of 15V.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,157
Location
Hickory, MS
Higher rails are not just for headroom. Sometimes things will perform better, e.g. a 12V signal might be of a higher fidelity with 24V rails instead of 15V.
Do you feel lucky?

Sometimes 24V rails on a circuit designed for 15V will release the smoke.

===
The resistor change the OP referred to is no doubt setting internal operating current correctly for the rail voltage.

JR
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,157
Location
Hickory, MS
I haven't seen anyone suggesting to do that.
When I was in the power amp business we paid a lot of attention to THD wrt approaching saturation... The power output specified with 3% distortion can look significantly better on paper (spec sheets) than power output with 0.1% distortion.

Different topologies and technologies have different behavior below saturation. Low NF tube circuits can start distorting well away from saturation, high NF solid state circuits generally get nasty quickly closer to clipping.

PS rail voltages is just another circuit design constraint to be managed by the designer.

JR
 

JW

Well-known member
Joined
Jun 8, 2005
Messages
894
Location
Portland USA
Very sorry about dropping my attention to this thread lo these many months ago. As to "why" I have a console that has +/-24v rails (Yamaha PM2000) and wouldn't mind being able to run 2520's. I think once long ago someone mentioned zener diodes in series to reduce local voltage rails at the opamp. But I don't think that worked from memory. This was years ago.

I'm not trying to incrrease headroom of the 2520, just adapt to my console.

It's not possible to add series resistance right at the opamp to bring the voltage rails down to say, +/-18V?
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,157
Location
Hickory, MS
The zener diodes in series is not crazy as long as they are followed by caps. The voltage drop from resistors will change with current draw so depending on where used and loading the resistors may not deliver a reliable voltage drop.

JR
 

Kingston

Well-known member
Joined
Nov 1, 2005
Messages
3,708
Location
Helsinki, Finland
If you absolutely must have a DOA, the APP992 should be superior to 2520 in just about every way, and while it's technically not a drop-in replacement it actually works just fine. At least in my applications which have been from input buffers to output transformer drivers - exactly where you would normally have a 2520.

And it does +-25VDC.

 

moamps

Well-known member
Joined
Sep 24, 2005
Messages
1,872
Location
Croatia, HR
From my experience, a standard 2520 type opamp designed with BC550/560 and BC141/BC161 or BD139/140 transistors will work without significant problems at +/- 24V, you just need to increase the emitter resistors of the output transistors (for example, from 5R6 to 10 or 15ohm).
 

Latest posts

Top