Adding direct outs to the Yamaha PM-1000….
Understanding the gain structure:
PM-1000 channel strip gain structure, as I understand it, and change it…
There may be some errors in this, and I’m SURE someone will have issues with my calculations, and changes, AND being human I may have made a few mistakes…
Input transformer:
The transformers is 300 ohm to 2.7 K. Therefore… 2700/300=9 so 1 to 9 is the impedance ratio. The square root of the impedance ratio, or 9, is the turns ratio of the transformer, that’s 3, so the voltage gain of the transformer is 3 and a voltage gain of three = + 9.542425 DB.
I have not taken the time to calculate the exact insertion loss of this transformer, but -1 to -1.5DB would be my guess. So for this exercise we shall give the transformer a total voltage gain of 8 DB.
First stage amplifier:
There are two main amplifier sections in each channel, along with the two echo sends. I’ll be talking about the main amplifiers not the Echo’s.
The gain, (which is set by the amount of negative feedback), is from 21 to 33 DB in main amplifier sections. Yamaha includes the transformer voltage gain in the calculation for the first stage gain, they call it 40 DB. I figure the amp is set at about 32 DB gain + the 8DB from the transformer and you get 40 DB.
No one is most likely interested in the padding, bypass and blending of the input with the out put of the first stage amplifier, however it is understandable, if, you want to take the time and take one switch at a time. For now let’s just say it works.
However, let’s take a look at the settings at -38 DB and up on the input selector switch.
With the input gain selector set at -38 the first stage output section still has R29, 30, & 31 for a total of… 10K from the output to ground, or a 10K load. That should pose no level reduction to the output of section one, as it is a very low z output.
At the input switch your straight to the transformer…at -38 to -60.
The only thing that controls the gain of the first stage from -38 to -60 is the feed back circuit. At -38 you have R19 at 330K in parallel with R21 at 5.6K, that works out to 5.5066K, or 5.5K.
-38 uses 5.5k in the feed back loop.
For another 6DB of gain switch to -44. At that setting R19, 330K & R22, 12K =11.5K.
For another 6DB of gain switch to -50. at that setting 330K & R23, 27K = 25K.
For another 10DB of gain switch to -60. At that setting 330K & R24, 100K =76.7K
So as the feed back resistance goes up the feedback goes down and the gain goes up…
Ok… I know most of you already new this feedback thing and a lot more, but some may not and that’s why I went through it.
From -60 to -38 the maximum output of this amplifier section is limited to -20DB by the input switching system. With the input switch set to -32 you can get up to -14 out. With it set at -26 you get -8 out. At -20 you’re at unity, zero gain. Everything above that gets padded down, and or, bypasses the first stage completely or in part.
So a maximum of 40 DB gain from the input to the EQ, and a maximum of -20 to the EQ.
The EQ is unity. According to Yamaha the gain of Q5 is consumed by the load of the EQ section which phase adds or cancels at a certain frequency based on the inductive capacitive filtering selected.
I’m not sure I agree with them on the gain thing. It may be that the circuit it self is totally unity because Q5 looks like a normal phase splitter to this guy. The gain of Q5 seams to just hold the positive phase above ground so that the center taped pots work as a voltage divider with the center tap to ground. When the boost or cut pot wiper is centered the inductive capacitive circuit is effectively grounded and the + & - phase are equally grounded through each half of the 50K pot. The reactance of the inductors and their series capacitors works out to around 1200 ohms for each of the three mid frequencies.
(At least that’s what I cam up with.)
You also have two resistors in each circuit and all that adds to the total load to ground of the circuit.
The EQ it self is a great design, but the frequency points are even harmonics of one another and basically trash the thing. To make it work right you need to change not only the caps but the inductors. Changing the caps makes it somewhat better but to me it’s not worth the time and money. Thing needs a total makeover!
Bypassing it does change the sound of the preamplifier.
Enough of that already….
Again these are my conclusions and I may be very wrong….
So we will go on to the channel fader, it cost the EQ section 6DB of gain but only because it’s on the emitter output of Q5. That’s the phase inverting high impedance end of the transistor. So if you keep the EQ and the fader you loose 6 DB.
So… first stage gain with transformer +40DB, Fader loss after the EQ is -6DB= +34 add booster amp at +21DB=56DB
That’s it you have a 56DB amplifier with a channel strip. Now add the output transformer at 600 to 600 with an insertion loss of 1DB and you end up with a preamplifier with 55 DB of voltage gain and 600 ohm output impedance.
55 db total!!!!!!!!!!!!
And… You will be running the preamp way beyond the sweet spot and it will sound weak… The higher the gain setting used the lower the headroom and the thinner the sound.
Your typical microphone preamp has around 68 to 75DB of total gain so this thing will not make a very good preamp as it sits. As for direct outs…. Again, you will be running the preamp way beyond the sweet spot and it will sound week…
So if you add direct outs to you PM-1000 that’s what you’re going to get.
A weak sounding preamplifier.
Typical direct out on a professional mixer has at least 60DB of gain and at +4 out.
With an input sensitivity of -60 you’re going to need at least 64DB total gain to get to +4
