Holmes Effects Loop Mod

             This mod is available for tube and solid state amps. It can be used for any amp as long as there is enough room to add it.

             The total labor and materials cost is $250.00 plus actual shipping. We never charge a “handling fee”.  Turn-around time is usually 6-11 business days.

              The power supply used can be either a resistor divider or a small proto-board with isolation transformer. Which one is used will depend on the style of your amp and room available.

             The circuit runs on 16-30 volts. The small effects loop board has its own precision regulator. The effects loop input is 6 megohms impendence at audio frequencies. Ron programs the input divider pad and the recovery gain on the effects loop depending on the position of installation. Loop style is 1/2 normal with non-breaking send and breaking return. Because quality jacks are used, this 1/2 normal design is reliable, quiet and solid. A DPDT switch for isolation can be added if desired for an extra $20.00

 

             As shown below, the effects loop can be installed in position A or position B.

 

1.        Position A is between the input jack and the input preamplifier.

2.        Position B is just prior to the amp phase inverter/power stage.

Position A loop mod location. Good for SS and simple, small amps.  The loop mod board programmed for approx. unity gain with some loss makeup. Input Z roughly 6-7 megohm at audio frequencies. The circuit is very transparent with little or no coloration or compression. Great for harp high-impedance boost for any impedance mics.  All signal wiring done with premium RG174/U coax.

Position B shows installation after two stock amp preamp sections are mixed before the phase inverter.  Since the loop out to an effects pedal needs to be at signal level or 25 to 100 millivolts audio, the sum of the two preamps is too hot. At the mix point (see above) the signal has to be padded to approx. 1/6 the level so as not to overload a typical pedal.

 

The gain following pedal out reinsertion back into amp has to be increased by same amount it was reduced so pedal would work without overload.. Goal is to duplicate original gain structure. That way the signal thru amp doesn’t jump or drop with effects pedal removal. Most pedals are roughly unity gain. The only load on preamp circuit is the resistive divider network as the loop mod  has very high input impedance.

 

The output is very low driving impedance to the amp’s next stage-either one more gain stage or the phase inverter.  The gain structure is analyzed so mod and amp are functional together. The Loop installation should be very transparent and unnoticed. This mod is good for a working stage amp that needs to incorporate a pedal or other outboard gear within a loop. If you have a cherry vintage Fender that has never been in a service shop; and you bought as a collector to put amp behind glass to add to your museum, this mod is not for you.

 

This mod is for working players who use real, often beat-up gear and want to use a pedal onstage. This mod will lower the value of your amp for collectible use. However, nearly all the vintage gear has been altered or carved up anyway so the mod has functional value. This for working musicians.

 

This loop mod board works well with solid-state or tube amps. I  mount board so as not to cover original graphics and make it neat. A nice film label above or below jacks is added for reference.  If board can be hidden with access to jacks, that’s best. See the Kalamazoo final pix. The mod is invisible unless you look up inside rear of amp behind the dress panel. I do my best to hide the modification.

 

 

Basic board schematic. Values are tweaked for individual gain structures or situations. Resistor values determine make-up gain following pedal re-entry. There is RF and noise suppression on board.

PC board is premium, FR-4 1/16 fiberglass with double-side, plated-thru holes for strength and durability. Premium parts for long , dependable life. Very sonically clean and open.

 

Shown also are typical power supplies used. I use whichever is most appropriate for your amp.  The loop mod board has it’s own regulation as well.

 

 

Shown at left is typical install in a typical tube amp. I use two series dropping power resistors  (R20 & R21) to improve heat dissipation and  safety margin from arc-overs.

 

I try to end up with low dissipation from the 1 watt Zener diode. Just enough current to keep it in regulation but way below maximum for safety.

 

All added components are securely mounted to a tag strip or turret post or spare eyelet. Work is solid, secure and sanitary. Wires tie-wrapped and neat. Photos show typical work.