PT-80 Delay & FX loop question

[dummysf]

i just built the pt-80 delay from general guitar gadgets, it works great.
Now i have a small problem :
I own a Tech21 TM120 amp,I plugged a delay  in to fx loop and when i switch it on the output level is very much lower. I contacted Tech21 support and this is what they answered me :
"...Actually, it's the input of the effect that needs to accept
line level signal (which is what's coming from the Effect Send, after the TM120's preamp section).  Pedals usually only accept instrument level (from the guitar's pickups), and that's why yours is "choking" the sound"

Now i want to know how to modify the pt-80 delay input that would accept line level signal.
in amp specifications is written that effect loop input impedance is 1M, and effect loop output impedance is 1K.

would be the correct sollution if i replace the 1M resistor between .047uF and pin 3 of TL072 with 1K resistor?Or is there another solution ?

thank you for answering

[nelson]

why make an effects loop line level......?

[object88]

Most rack effects take line level inputs.  I'm guessing that amp companies commonly think that in an effects loop, you'll typically use rack stuff, while pedals will come before the amp.

[Jaicen_solo]

I'd imagine it's the output level that needs attention. I had a similar problem with my Flextone II, which also wants to see a line level signal.
Basically, what you need to do is create a variable gain input buffer to give 0db, and a variable gain output buffer to provide a signal level of +4dB. A fair amount of gain, but probably achievable with a single dual opamp.

[d95err]

The Adjusticator at Geofex could solve your problem. It's a simple clean preamp that can convert from instrument level to line level.

[Scott Swartz]

The PT2399 delay chip can not handle line level without distorting, I discussed that in the tech paper for the PT-80.

What you have to do is reduce the signal level coming into the circuit by 5:1 and then boost it by the same amount leaving the PT-80 circuit.

You could add a simple resistive pad to handle the issue at the input.  You need 2 resistors forming a pad.  You could use the board location for the .047 cap, put a resistor there instead.  That new resistor and the 1 Meg resistor shown on the schem become your pad.  The new resistor is an 820K, and the 1 meg gets changed to a 220K.  You still need the .047 cap in series between the input jack and the new "pad", so you could mount the cap on a terminal strip or something off the board.

On the output, the last opamp stage that mixes the dry and wet could be changed to give a 5X gain.

On the schem look above the opamp you will see

1K series with .015 uF
10K
47 pF

Change these to

5K series with .003 uF
50K
10 pF

Now the stage will have the correct EQ characteristics as well as a gain of 5.

[dummysf]

thank you for your answers.
I made a test and do the folowing :
wired fx send-->ibanez tm5 --> pt-80 delay --> fx return

on ibanez tm5 i set gain to zero and then adjust level so that  output sound has the same level then when the effect loop is bypassed. output level on TM5 is set almoast to maximum. Now here is the part when i get confused.
If fx send is line level which is much higher than guitar level + it goes through tm5 and get amplified wouldn't be normal that there would be increase of volume or it would sound distorted instead of volume drop  and a nice clean sound ?



p.s. sory for my poor english

[scratch]

this is good stuff ... I built my rack effects to line level, and was looking at the PT-80, not even thinking about the issue of input level ...

[Scott Swartz]

You need to isolate the problem.

Remove the TM5 from the chain and try it.

The PT80 is unity gain (gain of 1) so if its working right, the level into the PT80 will be exactly the same as the level that is leaving and will be input to the effect return.

The only problem is the delay signal will be distorted if you don't do the mods I discussed above.  For the dry signal, the PT80 circuit will handle line level no problem.

If you are still getting a volume issue, there must be an issue with the PT80 or possibly even the amp.

[Headshot]

Sorry to butt in....

Scott, could you contact me at DSDesignsfx@hotmail.com  

I have a question or 2 relating to the PT-80 circuit.

Thanks!!

H

[dummysf]

guitar--> pt80 --> amp : sound is ok, volume level stays the same

fx send --> pt80 --> fx return : sound is ok but delay sounds better then plugged before, delay is not distorted but there is a huge volume drop when pt80 is on, when it's bypassed volume increases back to normal

fx send --> TM5 -->pt80 --> fx return : sound is ok, volume drop adjusted with TM5 output level
fx send -->pt80 -->TM5 --> fx return : sound is ok, volume drop adjusted with the same TM5 output level

and i still don't know what to do....

[sean k]

Whats happening is the output of the pt 80 is sitting on a 1M resistor and the fx return input is most probably sitting at about 100k so the pt 80 is being loaded by the low impedance it sees.So you need to lower the output impedance of the pt 80 to about 10k or so and this could be done by adding an emitter follower to the output like the one feeding the sa571,the 2n5088.Take the design from the 1uf cap ahead of the 100k bias resistor,take out all the RC filters and the feedback cap,.001 emitter to base,and where the 2n5088 1uf output cap goes to the sa571,put in a 25kA pot and thats your low impedance out.
 So from the 1M output resistor put in another 1uf cap and bias base of another 2n5088 with 100k,collector to 12V,emitter with 10k to ground and 1uf cap to top of 25k pot.All this mounted on an extra switch for low / high impedance output...or just keep using the tm5.

p.s. I'm not real hot on transistors but it should work.

[Scott Swartz]

The PT80 output is low impedance already since its output is from an opamp, so Sean's idea should not be required.

The line level gear that would "normally" be hooked into the loop will have likely have an opamp stage very similar to the PT-80.

However, there is one possibility that might be addressed by implementing an output buffer as Sean suggests.  Perhaps there is some sort of interaction of the feedback loop on the PT80 output and a feedback loop on the amp input.  Having a buffer may "break" the interaction.

The reason I say this is the TM5 likely has an emitter follower output and it does seem to behave differently with the TM5 in the chain.  The signal; will go through the TM5 buffer regarding of whether the TM5 effect is "on", since it is an electronic switching type of pedal as all Boss Ibanez etc are.

So, I don't agree with Sean's analysis, but the change he suggests might help.

Another thing to try is insert a 1K series resistance between the PT-80 board and the output jack.  This will help minimize the interaction discussed above also.

Its possible the volume drop is because of an ultrasonic oscillation that you can't hear but is reducing the percieved level.   The series resistance or buffer may stop the oscillation.

[sean k]

Thankyou for that Scott,I'm not really up on impedance and thought the 1M resistor set the output impedance for the PT-80...but this is how one learns,say what you believe and acknowledge the consequences.I've got the chips myself and will eventually put it together so this kinda stuff really helps.

[dummysf]

thank you all  for your answers. I'll try with 1K series resistance if that doesn't work, i'll try with adjusticator from Geofex. I'll report the results