Topic: New TS-2 Preamp ... under development (part II)

[jerryfreak]

I dont think demand and price point have anything to do with each other. I build by hand 20-30 preamps a month + microphones... A product is in demand because its good not because its cheap especially in a market where the consumer knows the difference.

::gets in back of line::

[fivefishdiy]

 



 

[Church-Audio]

 

My apologies I meant no disrespect to you or your product and I did not mean to derail your thread.

Chris

[sexysally]

More transformer talk pleas. 

[stevetoney]

^^ OK, in response to a couple people that are still curious...and because I still have additional curiousity about preamps and some things that you said...I'll ask another noob electronics question.  Following is quoted your response in an earlier thread and question follows...

1) you said that transformers reduces capacitors...I guess that's a good thing from the standpoint of less parts needed in the design (less to solder onto a board and less that can go wrong)

#1. Transformers block DC voltage (from the phantom power) going to the mic pre stage.  Because DC cannot travel from the primary winding to the secondary winding... only AC voltages can do that (which is the mic signal in this case.)  A tiny capacitor can do the same thing, DC blocking.

Yes, you have one less capacitor to solder but trafos bring its own kind of baggage. Trafos are expensive, bulky, heavy. And in a market where users are price conscious, trafo-based gear is usually out of the question.  So while a trafo will get rid of that small capacitor, it brings with it it's own set of "cons.".... $$$ of the part being the biggest thing. The bulkiness of it demands a bigger case, which is more $. The heaviness of it adds shipping weight, which adds to shipping cost, which is more $.

If the market is willing to pay for it, then trafos are used.

For those of us that view electronics as mysterious boxes in which magic happens inside, your response sparked additional curiosity.  So you said that transformers blocks DC voltage from going to the mic pre-stage.  Is that to say then that microphone phantom power is DC voltage and the rest of the pre-amp operates off of AC?

If that's the case, then I'm curious...if there's no transformer in many circuits, how does the DC power from a battery get switched to AC to power the preamp? 

Actually this seems totally wrong to me, but I'll leave the question as it stands and suffer the consequences of appearing to be really stupid in this thread.   

[Scooter]

super simplistic answer: no A/C(for power anyway) in there, its all D/C.  you want to block D/C from entering or leaving the preamp on signal lines(except for P48 on the input).  to further complicate things, the mic signal is actually A/C.

Edit:  Chris says it better below....

[Church-Audio]

^^ OK, in response to a couple people that are still curious...and because I still have additional curiousity about preamps and some things that you said...I'll ask another noob electronics question.  Following is quoted your response in an earlier thread and question follows...

1) you said that transformers reduces capacitors...I guess that's a good thing from the standpoint of less parts needed in the design (less to solder onto a board and less that can go wrong)

#1. Transformers block DC voltage (from the phantom power) going to the mic pre stage.  Because DC cannot travel from the primary winding to the secondary winding... only AC voltages can do that (which is the mic signal in this case.)  A tiny capacitor can do the same thing, DC blocking.

Yes, you have one less capacitor to solder but trafos bring its own kind of baggage. Trafos are expensive, bulky, heavy. And in a market where users are price conscious, trafo-based gear is usually out of the question.  So while a trafo will get rid of that small capacitor, it brings with it it's own set of "cons.".... $$$ of the part being the biggest thing. The bulkiness of it demands a bigger case, which is more $. The heaviness of it adds shipping weight, which adds to shipping cost, which is more $.

If the market is willing to pay for it, then trafos are used.

For those of us that view electronics as mysterious boxes in which magic happens inside, your response sparked additional curiosity.  So you said that transformers blocks DC voltage from going to the mic pre-stage.  Is that to say then that microphone phantom power is DC voltage and the rest of the pre-amp operates off of AC?

If that's the case, then I'm curious...if there's no transformer in many circuits, how does the DC power from a battery get switched to AC to power the preamp? 

Actually this seems totally wrong to me, but I'll leave the question as it stands and suffer the consequences of appearing to be really stupid in this thread.   

The transformer in this case blocks the DC from entering the input stage of the preamp. Normally you would use dc blocking caps to do this. The whole circuit operates from DC. The only ac part of the equation is the input signal it self. Transformers also tend to increase the CMR or common mode rejection of the input stage making for a quiet preamp design.

Sometimes you use a transformer with a higher ratio to also increase the gain before the first solid state stage to help reduce the need for added gain in later stages. That is another often not talked about advantage to transformers. A transformer is like a speaker in the sense that it transfers energy thus it changes the sound of the input signal by varying degrees, a transformer transfers it from one set of coils to another via induction. This also causes a transfer function effect where the transfer of signals from primary coil to secondary is not linear. Thus the flavor aspect comes into play. This is the main reason why some preamps sound better then others. Because solid state electronics by them selves are pretty linear by design. Transformers add that color to the signal that is hard to get in plain old solid state circuit design.
Chris

 

[Gutbucket]

[several responses answered the question while I was typing, but here's a bit more detail]

Although I should defer the to builders, I'll take a swing at answering your question to exercise my brain.  If I get a detail wrong, please correct me.

There is the signal from the mic which is a fluctuating AC voltage and there is power to operate the mic and preamp circuitry which is a steady DC voltage. Usually those are kept separate though the entire recording chain and that is the case for the recorder, the preamp and for non-phantom or plug-in powered microphones.  Dynamic mics don't need power to operate and tube microphones have their own dedicated power supplies and multi-pin cables that keep the signal from the mic and power to the mic separate.  DC voltage present on the A/C signal is called a DC offset and designers use a capacitor in series with the signal or a transformer across it to eliminate any DC offset that may be present in the AC signal between each stage or component (mic>preamp>ADC).  Phantom power and plug-in power is a way of powering condenser mics by combining the normally separate DC voltage to power the mic with the AC signal from the mic in the mic cable.  Both the power and signal share the same wires from the mic to the preamp.  Instead of the AC signal waveform fluctuating around 0 volts by going positive, then negative, then positive again, it fluctuates around the DC powering voltage of say +48v.  So the the voltage in the cable goes up a little above +48Vdc and down a little below +48Vdc. At the preamp side, after the point were the DC phantom power is applied to drive the mic, that DC component is removed or 'blocked' with a capacitor or transformer, leaving just the AC signal fluctuating around 0 volts again heading into the preamplifier circuit.

[Todd R]

Well, everyone's already got this one covered and I should just keep quiet probably, but in case it would help some people hear it all in hopefully more simplified terms:

-the preamp operates on DC voltage, which comes from the battery
-a condenser microphone needs to operate off DC voltage, which is provided by the preamp as phantom power (+48v DC)
-the microphone sends out an AC signal, which is used to capture the audio as a voltage signal

What happens is the preamp sends out say +48v DC voltage to the mics to allow them to work.  The mics then capture the sound as an AC voltage and combine that AC voltage with the +48v DC voltage that the preamp has already put on the lines (the mic cable).  When the voltage signal returns from the mics to the preamp, as a combined AC + DC voltage, the preamp must strip off the DC voltage since it only wants to amplify the AC voltage (the audio signal) and send that alone off to the A/D converter or recorder. 

The two general means used to strip off this DC voltage is to use capacitors or to use transformers, both of which will block DC voltage from passing through but will allow AC voltage to pass through.  Capacitors or transformers also have other properties and other effects on the sound which have been discussed above, but to meet the need of blocking phantom power/DC voltage, a preamp will typically make use of one of these two options.

[fivefishdiy]

Thanks for jumping in and answering guys!

This remains unanswered so I'll give it a shot.

how does the DC power from a battery get switched to AC to power the preamp?

First, I think we need to straighten some things up.

The amplifier/preamp circuit operates on DC voltages. This voltage/power is supplied by a battery or a DC power supply unit.

The miniscule AC signal you have at the input, think of it as a "control" signal.  That's all it is.

To really understand what is going on, you have to go back to the basics... in this case, all about Transistors. 

You may be thinking... But we're not using transistors. We're using IC chips... opamps to be exact. Well, Op amps are just a bunch of transistors arranged in a specific way.

So going back to basics, a transistor device allows you to control the large flow of current at it's collector by feeding it a small input current at it's base terminal.

The relationship between how much Collector current you get, by how much base current you feed to it is called the Current gain Hfe of the transistor. For some transistors, this could vary by 10x to 100x or even more.

Still with me? Let's simplify things. Let's look at a single transistor.

  The Base is connected to your mic signal. The Collector is where you get your output. The Collector gets it's power source from a battery.

So now, the varying, miniscule input voltage (coming from the mic) signal is fed to the base of your transistor which in effect causes a varying but larger voltage change at the collector of your transistor (which is where you get the output).

So what's really happening here is this... your preamp's output voltage comes from the DC power supply.  (Not from the inputted ac signal.)

But  now you're probably more confused... you're probably thinking if the output voltage is supplied by the DC power supply, how did it become an AC signal then?

Ahhh. because this varying DC voltage induced at the collector goes to another output capacitor, or output transformer.  <--- I bet you didn't see that coming.

What happens next is the DC component at the output is blocked, and only the varying voltage level is passed through by the output transformer or output capacitor.  Except this time, we're talking about voltage levels that are much higher!  But nevertheless, almost an exact copy of the input signal!   (Remember the Hfe, Current gain capability of the transistor we discussed above?)

The above is just a simplification of things, but that's the gist of what happens.

So by knowing the above things, you can see how

a) Any hum on the DC supply voltage will show as hum on your output signal.

b) The maximum output voltage capability of your preamp is dependent on your DC supply voltage.

c) If you feed a large input signal to your amplifying device, and multiply this by the gain of the preamp, but there is not enough DC supply voltage to properly represent that input signal at the output, then you get a flat top waveform, i.e. maximum reached (see note B above). .... we call this "clipping"

OF course, there are some tricks to get around some of these problems.

*You can get rid of the output capacitor by using DC Servos. (which is what I'm using on the TS-2)

*You can further increase the output signal for "free" (i.e. not dependent on the DC supply voltage) by using an output transformer with a 1:2 ratio or higher.

*You can use DC-DC converters so that you can convert a small DC supply voltage (say 9V or 12V) to a higher DC supply voltage for your preamp, thus giving you more headroom and more maximum output voltage capability. (which is again, also what I'm doing with the TS-2 preamp)

Hope this explains a little some of the things happening, of how/why it's happening.

[Roving Sign]

What exactly is a "DC Servo" - I see that referenced on some of my gear inputs...

[fivefishdiy]

What exactly is a "DC Servo" - I see that referenced on some of my gear inputs...

It's basically an opamp configured as an "integrator".... as in "integrals", remember calculus?

The simplest explanation is it cancels out the "constant" DC voltage offset at the output of the preamp stage by using an integrator circuit to feed a control voltage back to the "reference" pin of the preamp stage.

Result? the constant DC at the preamp output is removed without using capacitors in the signal path.

Why do we want to remove the DC anyway? Because they reduce dynamic range that is available for your preamp. Remember, your preamp has a maximum output capability (limited by your DC supply voltage). If the "baseline" of your AC signal output is at some DC level above the "0" mark... you're basically eating up into your headroom, and limiting your headroom and dynamic range.  i.e. you're going to clip early.

Reality is, with today's excellent manufacturing and chip designs... the DC offset voltages are really really low. We're talking like millivolts, microvolts here. 

So why don't all big manufacturers use DC servos? Well... maybe because it adds to the cost. You have another opamp chip and support components, and it eats up board space, and makes everything just complex. You have another active component that might break down, more power consumption, etc,.... when a single capacitor costing pennies can do the same job of removing DC. (like what I said, chips nowadays have excellent DC offsets... almost negligible... so maybe a capacitor will be just fine! I did listening tests to my pre, and it doesn't make a difference I can tell whether I'm running DC servo or capacitor coupling. Either ways, I provided a jumper for both options.)

But you know what, I'm not a big manufacturer. I don't sell hundreds of thousands or millions of units. So spending a few extra dollars for the DC servo instead of pennies on capacitors won't mean a potential loss of millions of $ of profit for me.

I can afford it 

[dactylus]

What exactly is a "DC Servo" - I see that referenced on some of my gear inputs...

It's basically an opamp configured as an "integrator".... as in "integrals", remember calculus?

The simplest explanation is it cancels out the "constant" DC voltage offset at the output of the preamp stage by using an integrator circuit to feed a control voltage back to the "reference" pin of the preamp stage.

Result? the constant DC at the preamp output is removed without using capacitors in the signal path.

Why do we want to remove the DC anyway? Because they reduce dynamic range that is available for your preamp. Remember, your preamp has a maximum output capability (limited by your DC supply voltage). If the "baseline" of your AC signal output is at some DC level above the "0" mark... you're basically eating up into your headroom, and limiting your headroom and dynamic range.  i.e. you're going to clip early.

Reality is, with today's excellent manufacturing and chip designs... the DC offset voltages are really really low. We're talking like millivolts, microvolts here. 

So why don't all big manufacturers use DC servos? Well... maybe because it adds to the cost. You have another opamp chip and support components, and it eats up board space, and makes everything just complex. You have another active component that might break down, more power consumption, etc,.... when a single capacitor costing pennies can do the same job of removing DC. (like what I said, chips nowadays have excellent DC offsets... almost negligible... so maybe a capacitor will be just fine! I did listening tests to my pre, and it doesn't make a difference I can tell whether I'm running DC servo or capacitor coupling. Either ways, I provided a jumper for both options.)

But you know what, I'm not a big manufacturer. I don't sell hundreds of thousands or millions of units. So spending a few extra dollars for the DC servo instead of pennies on capacitors won't mean a potential loss of millions of $ of profit for me.

I can afford it 

Thank you - looking forward to acquiring one of your new pre's!!

 

[fivefishdiy]

Just a quick update....

I'm trying to explore the possibility of using LASER engraved panels. This is a flatbed scan of a RED panel sent by a laser engraving company to me. I can't wait to get my hands on the actual panel. He said it turned out really nice.

So we'll see.



When I get the sample, I'll mill the holes using my in-house CNC.... this is going to be great.

I'll try to have other colors laser engraved next... maybe a dark blue or black anodized.

[flintstone]

Is it my imagination, or is the "R" in "TRIM" backwards?
Maybe using the Cyrillic alphabet font in the laser machine?