Pedal impedance question

ICBM

clarkefan said:

Is this why several Boss pedals on a board causes a small loss in volume, the loss you've mentioned several times?  The addition of multiple really small losses as described earlier by Sporky?

If I understood Sporky right each "1k output into 1meg input" scenario would mean a 1/1001 = 0.099% loss in signal, which is really small.  And all other brands would be equally affected.

No - the Boss pedal loss is caused by using a specific type of buffer called an 'emitter follower', which inherently has a gain of very slightly less than unity. This is inside each pedal itself, not how it interacts with other pedals. It's used because it's simple and cheap, only using one transistor.

Many (probably most) other buffered pedals use this simple form of buffer and will have the same problem. Some may have better-designed buffers, probably using an IC in a 100% negative feedback mode, and won't suffer from it.

ecc83

The worse-case output resistance of a pot is 1/4 of track resistance at 6dB (1/2) attenuation. This assumes the pot is driven from a near zero impedance source which it often will be if that is an IC and very low if a transistor emitter follower.

The latter were I assume used in early pedal designs because op amps were quite expensive. Now with SMT chips the component count for an IC 'follower' is less than for a discrete transistor and probably 1/2 to 1/10th the cost.

Despite the slight loss of gain I guess Boss think "if it ain't broke......"?

Dave.

ICBM

ecc83 said:

Despite the slight loss of gain I guess Boss think "if it ain't broke......"?

This must be the explanation, since there are several other known faults with their pedals (most obviously the SD-1’s bypass bleed-through and the GE-7’s excessive noise) which would be a piece of cake to fix, but haven’t been in 30 years of production... so presumably it’s more like if it’s broke, but not so badly that it loses sales, don’t fix it .

clarkefan

Can I ask another question?  I've been looking at impedances of my pedals and my second pedal (after TU-3, 1kohms output) is a Dunlop Clyde McCoy wah which apparently has an input impedance of 70kohms.  Does that mean my signal loss is already pretty serious before I get to the next pedal?  70k is a lot less than 1meg

ICBM

clarkefan said:

Can I ask another question?  I've been looking at impedances of my pedals and my second pedal (after TU-3, 1kohms output) is a Dunlop Clyde McCoy wah which apparently has an input impedance of 70kohms.  Does that mean my signal loss is already pretty serious before I get to the next pedal?  70k is a lot less than 1meg

Yes, but it’s still more than 10 times 1Kohm, so it will make no difference as long as the TU-3 is first.

clarkefan

Thank you ICBM

ecc83

ICBM said:

ecc83 said:

Despite the slight loss of gain I guess Boss think "if it ain't broke......"?

This must be the explanation, since there are several other known faults with their pedals (most obviously the SD-1’s bypass bleed-through and the GE-7’s excessive noise) which would be a piece of cake to fix, but haven’t been in 30 years of production... so presumably it’s more like if it’s broke, but not so badly that it loses sales, don’t fix it .

And if they DID tart up the GE-7 and give it a -100dBu noise floor you can bet your sweet A some cork sniffin' widdler would complain that "it does hot have sound of the old ones".

You CANNOT win with mooosicions!

Dave.