Feeler: EZ Tube Lunch Box

[G-Sun]

Ian: Do you envision the EZ optical compressor as 1 or 2 module width?

 

[G-Sun]

The regular EZTube amplifiers will happily drive 2400 ohms. That is exactly why I normally use a 2K4:600 transformer to drive 600 ohm loads. However, not many people use actual 600 ohms loads these days. They are mostly 10K bridging. Since the amplifiers will drive 2400 ohms you can safely drive four 10K loads at once via a 600:600 transformer.

I'm planning feeding first channel (using preamp)
to second channel (using eq and comp)
as an alternative to insert for mono tracking.
Will I have problems with the drive if patching it into the other directly?
(as the input xfm is 200ohm)

 

[G-Sun]

So,
I've made this module-signal-flow chart for my build:

 

[G-Sun]

And been going through the gain-staging

 

[G-Sun]

The pad switch -20db before 2nd gain stage:

I'm thinking a SPDT switch with label +20db.

I can't seem to need a fine-gain pot here,
and an option to utilize all available gain seem natural, though if it doesn't affect sound in a usable way,
then the only need for the extra gain would be if using a ribbon-mic.

And -20db here seems to give me 0db signal flow for the eq,
but I don't know how important that is.

 

[G-Sun]

The line levels are -6 to +47db
something like -10 to 40db seems better.

Maybe changing line in pad from -33db to -40db?
to  get -13db to 40db?

 

[ruffrecords]

Almost done my PSU. Question: where can I pull power for the relays for four Classic preamps?
Edit: could I simply use an off-the-shelf 12vdc supply?

Yes you can use an off the shelf 12V supply.  The pins on the 32way connector marked RELAY are intended for this purpose. If you use the backplane, these pins are buses so they supply all cards plugged into the backplane.

Cheers

ian

 

[ruffrecords]

Ian: Do you envision the EZ optical compressor as 1 or 2 module width?

At present I envisage the simple mono optical compressor being a 1 module width and 3U high.

Cheers

Ian

 

[ruffrecords]

The regular EZTube amplifiers will happily drive 2400 ohms. That is exactly why I normally use a 2K4:600 transformer to drive 600 ohm loads. However, not many people use actual 600 ohms loads these days. They are mostly 10K bridging. Since the amplifiers will drive 2400 ohms you can safely drive four 10K loads at once via a 600:600 transformer.

I'm planning feeding first channel (using preamp)
to second channel (using eq and comp)
as an alternative to insert for mono tracking.
Will I have problems with the drive if patching it into the other directly?
(as the input xfm is 200ohm)

Not quite sure I understand. You are not planning on feeding into the MIC input of the second channel are you?

Cheers

Ian

 

[ruffrecords]

So,
I've made this module-signal-flow chart for my build:

Looks good to me.

Cheers

Ian

 

[ruffrecords]

The pad switch -20db before 2nd gain stage:

I'm thinking a SPDT switch with label +20db.

I can't seem to need a fine-gain pot here,
and an option to utilize all available gain seem natural, though if it doesn't affect sound in a usable way,
then the only need for the extra gain would be if using a ribbon-mic.

And -20db here seems to give me 0db signal flow for the eq,
but I don't know how important that is.

In your signal level table you appear to assume the two amps have fixed gains of 40dB and that in the mic pre the gain switch changes the attenuation. This is not correct. In both amplifiers, the gain of the amplifier is directly variable from 3 to 40dB. In terms of calculating overall gain and levels, the two are equivalent, but in terms of noise they are completely different.

The noise at the output of an amp depends on the gain. The higher the gain the higher the noise. In my amps, with the gain at 7dB, the noise at the output is below -80dBu. With 40dB gain, the noise is about -60dBu. So if you set the second amp gain to 40dB, its noise will be -60dB not matter what happens before it, which is generally not acceptable. With the gain at 20dB to make up for an EQ, the noise will be around -80dBu which is OK. If you increase it to 30dB an have a switched pad before it, the noise will be around -70dBu which is porbably OK.

Cheers

Ian

 

[ruffrecords]

The line levels are -6 to +47db
something like -10 to 40db seems better.

Not quite sure what you mean. Are you talking levels or gain?

Cheers

Ian

 

[G-Sun]

Ian: Do you envision the EZ optical compressor as 1 or 2 module width?

At present I envisage the simple mono optical compressor being a 1 module width and 3U high.

Ok, but linkabel?

 

[G-Sun]

The regular EZTube amplifiers will happily drive 2400 ohms. That is exactly why I normally use a 2K4:600 transformer to drive 600 ohm loads. However, not many people use actual 600 ohms loads these days. They are mostly 10K bridging. Since the amplifiers will drive 2400 ohms you can safely drive four 10K loads at once via a 600:600 transformer.

I'm planning feeding first channel (using preamp)
to second channel (using eq and comp)
as an alternative to insert for mono tracking.
Will I have problems with the drive if patching it into the other directly?
(as the input xfm is 200ohm)

Not quite sure I understand. You are not planning on feeding into the MIC input of the second channel are you?

It would typical be like this:
Mic > Mic input strip 1 (preamp, bypassing eq and comp) > line out
> Line in  strip 2 (utilizing eq and comp) > line out > AD/DAW
(this would give me the wanted insert, for filters and loopback from DAW)

 

[G-Sun]

In your signal level table you appear to assume the two amps have fixed gains of 40dB and that in the mic pre the gain switch changes the attenuation. This is not correct. In both amplifiers, the gain of the amplifier is directly variable from 3 to 40dB. In terms of calculating overall gain and levels, the two are equivalent, but in terms of noise they are completely different.

The noise at the output of an amp depends on the gain. The higher the gain the higher the noise. In my amps, with the gain at 7dB, the noise at the output is below -80dBu. With 40dB gain, the noise is about -60dBu. So if you set the second amp gain to 40dB, its noise will be -60dB not matter what happens before it, which is generally not acceptable. With the gain at 20dB to make up for an EQ, the noise will be around -80dBu which is OK. If you increase it to 30dB an have a switched pad before it, the noise will be around -70dBu which is porbably OK.

Ah, thanks! this is very clarifying. I'll rethink some aspects then

So, that means not normally utilizing the last 10db of a gain stage?
and
that using the last 20db of second gain-stage for preamp is pretty pointless as it'll just amplify the noise from stage1.
Eg. for +80db, it'll be -60db noise from stage 1, amplified by second stage to -40db noise, right?

 

[G-Sun]

Another meaningful way to think of the second gain stage would maybe be as
eq gain adjust,
with a pot, labeled from -10db to +10db, center detent to 0db.
and bypass set to -30db.

But it would need some special wiring with a TPDT switch  (for toggling with eq bypass) and give -70db noise from second gain stage.

..just thinking loud

 

[ruffrecords]

Will I have problems with the drive if patching it into the other directly?
(as the input xfm is 200ohm)

It would typical be like this:
Mic > Mic input strip 1 (preamp, bypassing eq and comp) > line out
> Line in  strip 2 (utilizing eq and comp) > line out > AD/DAW
(this would give me the wanted insert, for filters and loopback from DAW)

OK, you are connecting to the line in of the second channel. The 33dB pad at the line in also raises the input impedance to about 6K6  (the pad consists of two 3K3 resistors and a 150R resistor) so it is very similar to a 10K bridging input and will not laod the output.

Cheers

Ian

 

[ruffrecords]

Ian: Do you envision the EZ optical compressor as 1 or 2 module width?

At present I envisage the simple mono optical compressor being a 1 module width and 3U high.

Ok, but linkabel?

Ideally, but as yet I do not have a scheme known to work.

Cheers

Ian

 

[ruffrecords]

Another meaningful way to think of the second gain stage would maybe be as
eq gain adjust,
with a pot, labeled from -10db to +10db, center detent to 0db.
and bypass set to -30db.

But it would need some special wiring with a TPDT switch  (for toggling with eq bypass) and give -70db noise from second gain stage.

..just thinking loud

The ideal answer would be to find a way to change the second amp gain by 10dB using a switch independently of the make up gain for the EQ. You normally set up the EQ by setting all the controls to flat and with the EQ IN you trim the seconf amp gain for overall unity gain from EQ IN to the OUT of the second amp. Then you switch the EQ out (and the pad IN) and adjust the pad for unity gain. The thing is, you don't know exactly what gain you have set for the second stage. It does not matter - all that matters is it equals the insertion loss of the EQ. To increase this gain by 10dB, the simplest thing is to ad d another resistor in parallel with the second stage preset gain pot via a switch - but you don't know what value to set this to. So....... one way to do this would be to switch in another preset pot and adjust this to increase the gain by 10dB. That way you always get the best noise possible. I normally recommend a 10K pot for the EQ gain setting preset because the mid-way point is 5K and this gives close to 20dB gain. If we make the new preset 5K then at its mid point it should add about 6dB of gain and you can tweak it until you get 10 more.

Cheers

Ian

 

[G-Sun]

OK, you are connecting to the line in of the second channel. The 33dB pad at the line in also raises the input impedance to about 6K6  (the pad consists of two 3K3 resistors and a 150R resistor) so it is very similar to a 10K bridging input and will not laod the output.

Yes, I was aware the pad-resistors. Just needed confirmation. Thanks!