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Increase input impedance of BGW 250B - request for help

R

ryanpg

New Member
I'm using a solid state BGW 250B power amp with a peavey rockmaster preamp as my guitar setup. I'm not sure, but I think I have an issue with matching impedances between the output of my pre and the input of the BGW

Preamp Out:
Load Impedance: 47K ohms or greater
Nominal Output: +10 dBV, 3v RMS OR +0dBV, 1v RMS - I'm currently using the 3v output

BGW 250D/E (don't have the 250B manual)
Input sensitivity: 1.4v for maximum output
Voltage gain: 26dB
Input impedance: Greater than 15K ohms

So to avoid "tone suck," don't I need the BGWs input impedance to be greater than the output impedance of the preamp? So like around 150K ohms or more?

Is there an easy way to increase an amps input impedance? Like replacing a couple resistors on the input chain? Maybe add a high/low impedance switch?

I'm looking at changing the values of R51 and R52. But that's really just a wild guess.

I have attached the schematic for the 250D/E but I don't know all the similarities or differences. I do know the 250B only has two 1/4" inputs - no XLR or octal.

Thanks!
 

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That's a little surprising that that the stated input impedance is "15k ohm". ...and the pre out wants to see 47k ohm or higher..

Plugging in the BGW may have an impact of the sound. can you rig the Peavey in it's normal way, listen to it, then plug in the BGW and see if the Peavey changes it's sound?

You could use a DI box from Sweetwater or wherever to buffer the preout feeding the BGW.

The BGW schematic is a little light to make any recommendations on how to change the input impedance. ... the input and amp sections are missing.

I doubt it would be a resistor change that would change the input impedance in a big way. The BGW are pretty old. They probably have an input circuit that makes the input impedance low-ish. I doubt they are using a differential amp front-end/input stage.
 
Jon_Logan, thanks much for your thoughts.

Jon_Logan said:
That's a little surprising that that the stated input impedance is "15k ohm". ...and the pre out wants to see 47k ohm or higher..
Click to expand...

Yeah, I was pretty surprised to find the BGW has an input load of only 15k ohm. That seems low even for line level to me, and instrument level is sometimes as high as 1M ohm, and most effects are at least 100k (I don't think BGW intended this amp to be used for instruments though).

Plugging in the BGW may have an impact of the sound. can you rig the Peavey in it's normal way, listen to it, then plug in the BGW and see if the Peavey changes it's sound?
Click to expand...

Well, the Peavey is a preamp, I don't have an "instrument" poweramp. But I can tell you there is some tone loss, sounds similar to plugging my guitar directly into my computer. Not as bad as I would have expected with such mis-matched impedances though.

You could use a DI box from Sweetwater or wherever to buffer the preout feeding the BGW.
Click to expand...

Yeah that was a thought I had, but one I'd rather not pursue if I can help it.

- rather not have a passive DI
- rather not have another box in the chain
- rather not have another wall-wart
- don't want to spend the $$$

Lol.

The BGW schematic is a little light to make any recommendations on how to change the input impedance. ... the input and amp sections are missing.
Click to expand...

Really? I'm no good at reading schematics, but at the "top" or "right hand side" of pages 2 and 3 of the attached PDF, I see "J5 Left input" and "J6 Right input" illustrated as 1/4" jacks. Or am I really that bad at reading schematics? :sigh:
 
The "Schematic" doesn't really show any circuitry. It is more of a switching diagram so I am not sure what comes after the input pots. I suppose you could try replacing R51 and R52 with larger pots but I am not sure how the following circuitry would react. You could put a simple buffer (jFET source follower or triode cathode follower) after the preamp.

Another concern; I am not familiar with that power amp. Are we sure that it is up to the task of guitar amplification (headroom etc).

mike
 
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By calculation, it seems that the amplifier input circuit Z is actually 47K, but the 22K gain control brings this down to 15K at the input jack when it's fully clockwise. So in theory, you could solve the problem by deleting the gain controls. Another possibility is to add 22K in series at the top of each gain pot, which inserts -6dB of attenuation, and switch to your preamp's higher output setting. That would bring you so close to matching the requirements that little further improvement could be expected.
 
Thanks BinaryMike, I appreciate the time spent on this!

I kinda get that what you're saying :) basically matched input impedance is close to ideal.

Well... here's a new twist - turns out the 250D/E is pretty much an entirely different circuit than my 250B. With different specs too.

Ugh.

They sent me the correct manual but it's tough to read. Now, the good news is, it looks like the 250B has an input impedance of 47k ohms, which matches the preamp. So the slight "tin can" sound might have it's origins elsewhere.
 

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Impedance matching is not the goal in this context. Ideally, you want actual power amp input Z to be greater than the minimum load spec for the preamp. Equal to the spec is certainly okay, but higher sometimes yields better performance.
 
BinaryMike said:
Impedance matching is not the goal in this context. Ideally, you want actual power amp input Z to be greater than the minimum load spec for the preamp. Equal to the spec is certainly okay, but higher sometimes yields better performance.
Click to expand...

That's the premise I started with. But, as I'm sure you can tell, beyond switching out resistors and caps, I'm pretty much useless. So now that I have the correct manual, I find the input impedance on the BGW (47K ohms) roughly matches the output impedance of the preamp (47K ohms or greater).

Now, there's a mod out there for the preamp that's supposed to help with this.

C30 (main outputs) C40 (effects send) coupling caps are too small, and may act as a low cut filter. This is noticeable if the unit is plugged into products with an input impedence below 100K ohms. result is a thin sound replace C40 & C30 each with a 1uF 400V film cap
Click to expand...

Peavey preamp schematic attached.

I'm waiting for a better scan of the 250B from BGW. BGW has been AMAZINGLY helpful in emailing me manuals!
 

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Generally speaking, in other aspects of audio, the generally accepted ratio of preamp out to power amp input impedance is 1:10, to minimize the interactions you're trying to avoid. Given that, you'd be looking for an amp with 470K ohms. Highest I've personally seen is 300K ohms on the balanced input to my Audio Research 100.2, although almost certainly there are higher.

FWIW, none of my Crown or QSC pro amps are remotely close to having that high of input impedance.
 
So I've been reading what everyone has said, and searching around the net trying to understand impedance transformers and matching impedance etc.
And, I've got to admit. I'm pretty lost.

I know that ideally, input impedance should be at LEAST 100x output of the device being attached.

Should I be looking to increase the input impedance of the Power Amp?

Should I simply use a High -> Low instrument DI box between the Pre and the Power?

Should I just buy an audio xfmr and wire it to the input of the Power amp?

My brain is getting scrambled. I need simple, layman's language.

:drool:
 
ryanpg said:
So I've been reading what everyone has said, and searching around the net trying to understand impedance transformers and matching impedance etc.
And, I've got to admit. I'm pretty lost.

I know that ideally, input impedance should be at LEAST 100x output of the device being attached.

Should I be looking to increase the input impedance of the Power Amp?

Should I simply use a High -> Low instrument DI box between the Pre and the Power?

Should I just buy an audio xfmr and wire it to the input of the Power amp?

My brain is getting scrambled. I need simple, layman's language.

:drool:
Click to expand...

I think you should just connect it and go from there based on the results. Too many times I see worry about a problem that may not even manifest itself.
 
whoaru99 said:
I think you should just connect it and go from there based on the results. Too many times I see worry about a problem that may not even manifest itself.
Click to expand...

I think you're right. I've had a look at a bunch of other guitar power amps. The kind that I think would normally be used with the peavey pre.

Marshall 20/20 input impedance: 20K ohms
Rocktron Velocity 100 Input impedance: 10Kohms
Peavey Classic 50/50 power amp: 250k ohms

So... eh... sleep for now. I'll start thinking again tomorrow.


Thanks!
 
ryanpg said:
So I've been reading what everyone has said, and searching around the net trying to understand impedance transformers and matching impedance etc.
And, I've got to admit. I'm pretty lost.

I know that ideally, input impedance should be at LEAST 100x output of the device being attached.

Should I be looking to increase the input impedance of the Power Amp?

Should I simply use a High -> Low instrument DI box between the Pre and the Power?

Should I just buy an audio xfmr and wire it to the input of the Power amp?

My brain is getting scrambled. I need simple, layman's language.

:drool:
Click to expand...

It's important to understand the difference between a preamp's output Z and its minimum recommended load Z. Output Z can be orders of magnitude lower than min load. Watch out for this distinction when reading spec sheets. If min load is given, then you're going to be fine with that value of load Z or greater. If output Z is given, then you have to think a bit. 100 times that value for the actual load is certainly safe, but not always achievable. Power amps exhibit a similar issue. For high damping factor, the designed output impedance is generally kept below 0.1 ohm, yet you typically cannot load the amp with anything below four ohms.
 
I'm no expert, but I don't see much of a problem here. The 47k is the min. load impedance, not necessarily even close to the source impedance of the pre, and the amp specs list 47k. Assuming BinaryMike is correct (and I would assume that), removing the gain controls certainly makes sense. You don't need them anyway.

Also, you shouldn't be using the 3V, probably. The amp sensitivity is only 0.6V in mono.
 
BinaryMike said:
It's important to understand the difference between a preamp's output Z and its minimum recommended load Z. Output Z can be orders of magnitude lower than min load. Watch out for this distinction when reading spec sheets. If min load is given, then you're going to be fine with that value of load Z or greater.
Click to expand...

BinaryMike - I think you hit the nail on the head. From the preamp manual:

Preamp Output (3 V RMS):
Load Impedance: 47 K ohms or
greater
Nominal Output: +10 dBV,
3 V RMS

Preamp Output (1 V RMS):
Load Impedance: 47 K ohms or
greater
Nominal Output: +0 dBV,
1 V RMS

dumptruck; said:
I'm no expert, but I don't see much of a problem here. The 47k is the min. load impedance, not necessarily even close to the source impedance of the pre, and the amp specs list 47k. Assuming BinaryMike is correct (and I would assume that), removing the gain controls certainly makes sense. You don't need them anyway.

Also, you shouldn't be using the 3V, probably. The amp sensitivity is only 0.6V in mono.
Click to expand...

Yes, thanks to everyone's patient help I think I do understand now. The preamp spec sheet only provides Load Impedance.

dumptruck - I think you missed that the original schematic I posted was wrong. The actual poweramp doesn't have gain controls. HOWEVER, thank you for suggesting I use the 1V output.

I did this, and whatever "tone" I was hearing as impedance mismatch is gone! Absent now are those overly bright transients (very obvious with a guitar pickup when the pick hits the strings). Using the 1V output didn't result in any noticeable volume drop either - so I'm excited about that.

I think for now we can call this thread "done." Hopefully, the schematics, the discussion and the resolution will be helpful to more people than myself.

Again, THANK YOU to all who helped answer.
 
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