Jet City - Jettenuator

kh602 · Post by **kh602** » 09 Jul 2014, 22:20

The jettenuator seems to be a fairly simple design, i was under the impression there
was more to it than just a purely resistive attenuator. Works quite well though.
Will draw the DI/MIC SIM up at a later date. Seems to be the only complex thing
inside.

twangcat · Post by **twangcat** » 14 Jul 2014, 23:59

I can't see any ground connections at the schematic - except for 16 ohm socket TN and all S. With jack inserted into 16 ohm input [no speaker to output] load resistors seem to be connected at only one side [16 0hm T] what means open circuit for an amp. In such a case load line schematic can't be complete - those resistors should be connected to ground somehow - but maybe I've overlooked something... By the way - some shots of the internal parts would be welcomed

kh602 · Post by **kh602** » 16 Jul 2014, 21:55

I havent posted or said much on it because i dunno how everyone/this site doesn't get shut down for ripping designs but i do it
a lot and reverse engineer stuff quite often anyway, i just dont post, however I thought i would see how it goes with this post. anyway..

the ground is at 16 ohm sleeve terminal yet it grounds the rest of the sleeve terminals off as connect/follow the trace. (16ohm socket is the only one with a switch lug on it, the rest 8 and 4 are just mono sockets) Ground in this case is the sleeve of the cables you use to complete the circuit
amp(ohm selector out) > cable > into jettenuttor (16, 8 or 4ohm) > cable > speaker cab. None of the ring terminals are used in the circuit/not connected. the ohm jack sockets are like the ones
used in guitars and the speaker and line out are normal pcb mount plastic trs sockets. The socket

I am not sure why the 16ohm tip switch is grounded...maybe its a safety thing in case the speaker blows or maybe to shunt and excess to ground

twangcat · Post by **twangcat** » 17 Jul 2014, 02:54

Thanks for your reply ! I didn't think about ground as an electric potential but rather as one of the two output transformer wires - the one connected to the socket sleeve I called [ground]. What puzzles me is lack of any resistance beetween that point and the tip of the jack - second [hot] output transformer wire. According to the schematic - from 16 ohm socket tip [hot] signal flows through 2 parallel 15 ohm resistors and next in series to another parallel pair - 15 ohm resitor and 16 ohm rheostat, and next ... nowhere [without speaker connected and ignoring 50K line out]. So - as a 16 ohm load box jettenuator would have about 15-16 ohm resistance IF that parallel-series circuit were connected beetween 16 ohm socket tip and any socket sleeve/ground line point. Simply I can't see this second connection. Of course there is a possibility that I need a pair of glasses...

mozwell · Post by **mozwell** » 17 Jul 2014, 06:30

The external speaker must be used, so all these resistors are in series with the speaker.
Simple resistor voltage divider.

The attenuator is not a dummy load box, however, if you plugged a shorting jack into the speaker out 1/4" socket, or just connected the tip switch to 0V (connected TN to S on the speaker out socket), the attenuator would be a dummy load if no speaker was plugged in.
As it stands, if you are using a valve amp, and the attenuator & forgot to plug a speaker in, instant no load on the amp, with all the problems that will bring to you.

twangcat · Post by **twangcat** » 17 Jul 2014, 12:57

And that's exactly what I think. Now - what to do with the original manual quote...
" 2. OUT SPEAKER - connect to your speaker cabinet to hear your amplifier as normal. Jettenuator is a full-load device, so it is not necessary to use this connector in all applications. Note a reduction in output level of approximately -3dB even with the level set at 9 is inherent to the design of Jettenuator. If a speaker is not connected, set the Jettenuator control at 2 to provide the proper ohm load on your amplier’s output. "
What's more - when using 4ohm input with shorted speaker out socket, we are in danger of shorting amp's output transfomer at one of the wiper's travel ends - not good...

Nocentelli · Post by **Nocentelli** » 17 Jul 2014, 13:13

kh602 wrote:I havent posted or said much on it because i dunno how everyone/this site doesn't get shut down for ripping designs but i do it a lot and reverse engineer stuff quite often anyway, i just dont post, however I thought i would see how it goes with this post. anyway.

Thanks for the schematic - I assume you drew it out yourself?

Off topic, but worth pointing out...

There is no reason this site or any individual should be at risk of being "shut down" for publishing new schematics for existing products. It is only copyright infringment to publish artwork (e.g. a schematic) where the copyright to that artwork is owned by an individual or company who objects to it being published (e.g. EHX factory schematics, the copyright of which is owned by EHX). Uploading an EHX factory schematic is a no-no here, and the moderators will pull it quickly. However, if I redraw my own schematic for the Big Muff, I own the copyright of that image and can publish it wherever I damn well like. Designers have no intellectual property rights over the schematic (i.e the arrangement of components that makes up the circuit), but they can object to their own artwork being used. Patents are a different matter, but pedal circuits almost never have such protection.

A couple of years ago, Lance Keltner from Retro Channel got our original ISP to shut down this site by using a spurious DMCA complaint to our ISP - He claimed we were infringing copyright and hosting loads of images we didn't own the rights to (when we weren't at all). The original ISP couldn't be bothered to look into whether the complaint had any merit, and just dumped us. We have since moved to a new provider that isn't likely to buckle under such fallacious tactics.

kh602 · Post by **kh602** » 19 Jul 2014, 23:12

sorry for the late reply. yes i drew it up myself. i will post pictures too now that everything seem cool.

okgb · Post by **okgb** » 20 Jul 2014, 16:24

I'd love to see the speaker sim part , doesn't look to be much and guessing the transformer is only balancing
the xlr output

kh602 · Post by **kh602** » 20 Jul 2014, 22:58

i have drawn it out in rough on paper but not drawn it up in eagle yet.
The only thing i have no idea about is the 2 sot-23 diodes. i think it contains 2 diodes
in series as the 2 outer pins are used in some kind of configuration like so

The mic sim is pretty cool. I hooked the jettenuator to my laney gh50l and mic sim into my
firewire soundcard and out of the active montors which was pretty much in stereo eg. monitors on my right amp on my left
and it sounded pretty damn close/the same. Just the mic sim needed more preamp to hear it better
but not too much otherwise it would be gritty sounding from the clipping of the sound card.

deltafred · Post by **deltafred** » 21 Jul 2014, 08:35

Do the SOT-23 devices have any codes printed on them?

apollomusicservice · Post by **apollomusicservice** » 21 Jul 2014, 11:43

Looking at these pictures, it seems to me that the one part of this schematic looks like this

twangcat · Post by **twangcat** » 22 Jul 2014, 01:35

Big Thank You to kh602 for the fine pictures !
I've tried to rearange the schematic using these, but this is rather guesswork, without any actual circuit measuring of course. Anyway - it MIGHT be correct, but there are some inaccuracies...
Jettenuator seems to have correct impedance [resistance] as a load box for dedicated 16-8-4 ohm inputs without speaker, but with the speaker connected those resistances are about 12.7, 6.6 and 1.95 ohms [with correct impedance speaker] - if my calculations are OK. Not so bad, though a little low - especially for 4 ohm input at full open setting.
16 ohm socket switch is used for connecting two parallel 15 ohm resitors to ground - lowering load impedance for 8 and 4 ohm inputs.
I have no idea why the full seen impedance [from the amp side] is assumed to be nominal at 2-3 setting of the big rheostat [according to manual].
The highest load impedance should be seen with the speaker [pot wiper] shorted to ground - speaker virtually out of circuit - and the lowest with the pot on full [speaker impedance in parallel with 15 ohm resistor and 16 ohm pot]. These settings have no meaning without speaker connected - why set pot with unconnected wiper to 2 - [ignoring line out/speaksim out signal availability] - ?
Obviously this argumentation needs to be verified and quite probably contains mistakes ...
For now I have no access to the device, but simple measuring input sockets resistances with and without speaker connected [various pot settings] should explain much.
Strangely there is no visible connection soldered to the 16 ohm socket's sleeve lug. The sleeve is probably connected via mounting washer to the conductive case/ground [if it works at all] but this is not the right way I think. A soldering mistake ...?
My schematic is arranged in somewhat unusual manner to ease comparing with the pictures and the earlier kh602 schematic version.

apollomusicservice · Post by **apollomusicservice** » 22 Jul 2014, 08:31

Complete schematic

Rolbista · Post by **Rolbista** » 20 Oct 2014, 15:36

Do I understand correctly, that in the last schematic what an amplifier sees is: two 8R resistors (so 4R impedance/resistance) followed by a 7,5R potentiometer (a 16R pot with a 15R resistors across legs 1 and 3)? So let's assume that the pot is turned halfway - in the end it's something like 4R+ 3,75R on the pot (I guess the taper is not linear but this is just a thought experiment) + remaining 3,75R going to ground = 11,5R on the 8R input. Isn't that incorrect?

apollomusicservice · Post by **apollomusicservice** » 20 Oct 2014, 18:36

Speaker impedance/resistance must also be taken into account.

phatt · Post by **phatt** » 21 Oct 2014, 02:50

Thanks Apollo good schem,, just adding a word about that big pot
I'd say by the Size,construction and label on the L pad it's NOT a pot so the schematic would look more like this.

I don't know about the 16 Ohm version but the 8 Ohm version measures ~40 Ohm on the top and ~10 Ohm on the bottom,having had a few of the 8 Ohm versions apart to measure the 2 separate windings these are not normal 8 Ohm pot.

The reason for the odd setup is to keep the Z relatively close to the speaker Z at any setting.
But used in this situation that may not be important,, better minds may have clues as to how it works.
HTH, Phil.

phatt · Post by **phatt** » 21 Oct 2014, 02:59

Adding a gut shot of the internals of L pads. Phil.

apollomusicservice · Post by **apollomusicservice** » 21 Oct 2014, 06:54

Phatt, you're right. Tnx

J0K3RX · Post by **J0K3RX** » 22 Oct 2014, 01:28

rheostat

Jet City - Jettenuator [schematic]