Twin Line Amp V2

I finally got around to updating the documentation for the V2 version of the Twin Line Amp. I have expanded it to include a basic circuit description and I have added more details of how to configure it for various applications such as passive EQ gain make up, passive and virtual earth mixing. You will find it in the TwinLineAmp folder of the DIY tab of my web site:

http://www.customtubeconsoles.com/diy

Cheers

Ian

Hi Ian,
i was wondering about the THD graph with swept frequency with RMA...
or level of distortion at 100HZ and 1KHZ with REW

kambo said:

Hi Ian,
i was wondering about the THD graph with swept frequency with RMA...
or level of distortion at 100HZ and 1KHZ with REW

Click to expand...

I have not made those particular measurements. Under what conditions do you think they should be measured?

Cheers

ian

ruffrecords said:

I have not made those particular measurements. Under what conditions do you think they should be measured?

Cheers

ian

Click to expand...

0db input at unity gain with REW would be just fine...  for 100hz and 1Khz

edit  :  if distortion is not measurable at 0db, may be higher input level...

kambo said:

0db input at unity gain with REW would be just fine...  for 100hz and 1Khz

edit  :  if distortion is not measurable at 0db, may be higher input level...

Click to expand...

The Twin Line Amp doesn't do unity gain - the minimum is 6dB but the test could still be done with 0dBu output. Unloaded the distortion will be very low.. Loaded with 600 ohms, (via an 2K4:600 output tranasformer) at +20dBu at 1KHz THD is only about 0.3%. 0dBu will be 10 times lower (0.03%) and unloaded lower still. At lower frequencies the transformer may begin to make a significant contribution.

Cheers

Ian

thanks Ian,

i have  couple more questions re unbalanced mixing....

• on TLA2 manual page 13 TLA2A Passive Bus MIxer with Master Fader drawing :
am i connecting 10K LOG pot to each of the MB2 to MB32... and all sums at IN1 ?
or all MB1 to MB32 connecting to one single 10K LOG pot

• page 15 Virtual Earth Summing Bus Amplifier
my understanding is i connect all my inputs via 47K to SUM1... so, whats happening to IN1,
is just not connected at all ?
also, how do i adjust the output level, since GAIN1 is omitted..

kambo said:

i have  couple more questions re unbalanced mixing....

• on TLA2 manual page 13 TLA2A Passive Bus MIxer with Master Fader drawing :
am i connecting 10K LOG pot to each of the MB2 to MB32... and all sums at IN1 ?
or all MB1 to MB32 connecting to one single 10K LOG pot

Click to expand...

MB1 is the unbalanced passive  bus. In the channels, outputs are connected to the bus (MB1)  via 47K resistors. In the passive summing TLA, MB1 gets connected to the 10K pot which becomes the master fader for that bus and then to IN1. As the TLA has two identical amplifiers, another bus can be connected to the second amp. So MB2 could be connected to IN2 via another (optional) 10K pot. If MB1 and MB2 represent left and right of a stereo bus then the 10K logs can be ganged to make a stereo master fader.

• page 15 Virtual Earth Summing Bus Amplifier
my understanding is i connect all my inputs via 47K to SUM1... so, whats happening to IN1,
is just not connected at all ?
also, how do i adjust the output level, since GAIN1 is omitted..

Click to expand...

As shown in the schematic, for virtual earth summing, IN1 is connected to 0V. It is just like an op amp summer where the + input (IN1) is connected to 0V. The - input is fed from the bus resistors. As in an op amp, there is no need to alter the gain depending on the number of feeds. The stage gain is unity (set by the ratio of the 47K feedback resistor (Rfb) and the 47K bus resistors. Unfortunately Rfb itself cannot be changed as it is part of the dc biasing network for the first tube. As with the passive mixer, the buses would normally enter on MB1 thru MB4. The 47K bus feed resistors would be on the channel cards. The downside with the VE mixer is that you cannot easily add a master fader. It has to come after the amplifier. If the output of the TLA feeds an output transformer then you could fit a 600 ohm balanced pot across its secondary as a master fader.

I will expand both the passive and VE sections to make it clearer what happens.

Cheers

Ian

thank you sir

ruffrecords said:

Loaded with 600 ohms, (via an 2K4:600 output tranasformer) at +20dBu at 1KHz THD is only about 0.3%.

Click to expand...

is this +20dBu/1KHz at output or input ?

kambo said:

is this +20dBu/1KHz at output or input ?

Click to expand...

Output.

Input level depends on gain. Distortion really only depends on output level (and load) - it is almost independent of gain. It is a little higher at max gain due to less NFB.

Cheers

Ian

ruffrecords said:

Output.

Input level depends on gain. Distortion really only depends on output level (and load) - it is almost independent of gain. It is a little higher at max gain due to less NFB.

Cheers

Ian

Click to expand...

so, in the case of using TLA as virtual earth summing bus amp,
and feed it with 20dBu/1KHz on one channel,
i should be able to measure more/less  0.3 % THD on output...
am i correct ?

kambo said:

so, in the case of using TLA as virtual earth summing bus amp,
and feed it with 20dBu/1KHz on one channel,
i should be able to measure more/less  0.3 % THD on output...
am i correct ?

Click to expand...

Configured as a VE amplifier the TLA has 0dB gain so a +20dBU input would appear at the output as +20dBu. However you will measure less than 0.3% distortion if you do not load it. The TLA normally feeds a 2:1 output transformer so +20dBu at the TLA output would appear as +14dBu at the output of the transformer.  If you then load the transformer with 600 ohms you will se about 0.15% distortion (6dB less because the output is 6dB less) To get +20dBU at the transformer output you would have to input +26dBu to the TLA. Then, if you loaded the transformer secondary with 600 ohms you would see about 0.3% distortion.

Cheers

Ian

thanks Ian,

when u say 2:1 output trx, can i use LL5402 wired as 2:1,
or do i need 2k4:600 trx as noted in ur docs.

kambo said:

thanks Ian,

when u say 2:1 output trx, can i use LL5402 wired as 2:1,
or do i need 2k4:600 trx as noted in ur docs.

Click to expand...

Unfortunately, the LL5402 is only specified for use in special semiconductor feedback circuits. It is unsuitable for use with tubes because it does not have enough primary inductance. You do need to use one of the transformers mentioned in my docs or you can also use a Cinemag Cm-2820.

Cheers

Ian

"LL5402 does not have enough primary inductance"

that explains....
i had LL5402 on some of my DIY tube gear, they always sounded nice but somewhat weird!
when i used RMA to fine tune them, i was always seeing nice THD figures, and flat frequency response...
then i start measuring THD at 50 Hz  woooo THD is off the roof 
in some cases more than 10%... and THD at 1KHz is still only 0.3%

i changed most with 10k:600  transformers! i am happy now!

ie: people loves slow blow mic_pre unbalanced outs.... yes, measure the distortion at 50Hz with
LL5402 on it... it sucks... my slowblow is unbalanced too now!

cut the story short. i have a dozen of  LL5402's to use on something else  ;D ;D ;D
sorry for the offtopic post