Preamp explanation please

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dukecv

Preamp explanation please
« on: 23 Oct 2017, 03:45 am »
Ok, call me an idiot, but I won’t know without ever asking so:

Referring to pre amps, how do I know which one would be right for me?
What is the difference between:

-passive pre (basically just a volume control, right?)
-phono pre (no idea) don’t listen to records
-line stage pre (see this on audio research stuff)
-buffer pre (what are we buffing here?)
-and then just a regular preamp

And then on top of this, I’m not sure how to understand the input/output resistances on the components, and how that comes into play. What works with what, and what doesn’t.

Also the input voltage on an amp and what my dac has for an output voltage. Somehow I’m thinking this would decide whether I could use a passive volume control or not.

My DAC is a Schiit Modi. Was wanting to buy a small wattage tube amp to go with my omegas, but also want to understand this before going any further.

Thanks

belle harbor

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Re: Preamp explanation please
« Reply #1 on: 23 Oct 2017, 12:10 pm »
Your DAC has an output of either 1.5 or 2 volts rms depending on the model.  This is comparable to that of a CD player.  The simplest way to get into tubes is to select an integrated amp (which has its own preamp stage built in) and feed your DAC into it.  The extremely low output impedance of the DAC, 75 ohms, makes it compatible with any amp that I'm aware of (both SS or tube).  The only time you would need to use any kind of passive level control or preamp is if you select a power amp and need some way to control volume level.  Personally, I don't see the need to go that route. 

As far as input and output impedance is concerned (again, not of any concern with your DAC), if improperly matched, there could be some frequency response issues depending on the capacitance of the interconnects.

Hope this helps you a bit.

randytsuch

Re: Preamp explanation please
« Reply #2 on: 23 Oct 2017, 01:00 pm »
Passive pre is just a pot or similar for vol control
Phono just for tt
I would expect line stage has some gain, but no phone but not sure
Buffer would have no gain.
Pre should have some gain. May have phono

Since you need vol control there MAY be an advantage by adding a buffer after the vc.  The vc will have a higher output impedance than the dac, if it's a pot or resistance based vc.

dukecv

Re: Preamp explanation please
« Reply #3 on: 30 Oct 2017, 12:57 am »
Thanks guys. So if we focus on a passive pre here for a sec:

Are potentiometers used in active pre’s?
And are attenuators users in passive pre’s?
What’s the difference between the two? As far as I can see, potentiometers have an ohm rating and attenuators don’t.

Elizabeth

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Re: Preamp explanation please
« Reply #4 on: 30 Oct 2017, 01:11 am »
Few other things..
A passive preamp may, or may not have problems with sounding lean. This is due to impedance mismatch, and the fact the passive pre may not allow enough power through, or the source does not HAVE enough power to push to passive.(I may not be using the right terms, but the results are right on)
It can be great, or bad, total crapshoot until YOU have it in the system to find out.

The buffer was originally made to separate circuits. so nothing is fed backwards.
The audio use has usually been in a buffered tape recorder line.
Stand alone buffers are made for audio now. Cheap, usually with one or two tubes. No gain. But they are powered. So the lean lack stuff of passive is not part of the deal. Usually they are dirt cheap and are a bit grainy. and once you are paying more.. you should just buy a preamp!

Line level.. All preamps without a phono stage are 'line level'. NONE used to be made before CD arrived. As digital took over, preamps without a phono built in became 'line level'
Some companies still use the nomenclature. other do not. Since most preamps do not have a built in phono anymore. (or only as an option)



jules

Re: Preamp explanation please
« Reply #5 on: 30 Oct 2017, 03:32 am »
Going back to your original question dukecv, it's normally accepted that the minimum ratio of amp input impedance to pre-amp output impedance should be at least 10:1 and preferably higher.

Wherever you have a volume control, you have a variable resistance. It doesn't matter what name it has but the range can be important. One pre-amp might have a 10k ohm variable resistance [low I admit] while another might choose a 100 kohm max. The maker of your chosen tube amp might give you some recommendations on a suitable range for the vol. control resistance.

A "regular" pre-amp will probably have a buffer stage and usually [though not always] have some gain.

phono stages compensate for the fact that the signal coming off vinyl, if you were to simply amplify it "as is" would lack bass.

So unless your DAC has a volume control, you'd probably be best off with a pre-amp that has volume control, some gain and buffering. The gain will probably help with your proposed tube amp and my guess would be, without knowing the impedance values of the equipment involved, that some buffering will also be of value.
« Last Edit: 30 Oct 2017, 05:08 am by jules »

Speedskater

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Re: Preamp explanation please
« Reply #6 on: 30 Oct 2017, 01:12 pm »
An 'attenuator' is a component that reduces or controls signal level.
An 'attenuator' may be fixed or adjustable. It may be stepped or continuously variable.
Some types and names for adjustable attenuators are pot, volume control, variable resistor and potentiometer. 

randytsuch

Re: Preamp explanation please
« Reply #7 on: 30 Oct 2017, 04:21 pm »
Types of passive volume control include:

potentiometer - alps blue velvet and tkd are considered good.  Uses resistance to lower the signal level.

stepped attenuator - acts like a pot in that it uses a resistance to lower signal level.  Difference is that it uses fixed resistors, instead of a wiper in a pot.  Usually considered a step up from a pot, and priced accordingly lol.
BTW, this is normally a rotary type device, where you turn it, and there are "stops" as you turn it.  Each stop selects a different resistor, and so is a different volume.
There are different types of stepped attenuators:
http://www.goldpt.com/attenuator_types.html
BTW, you can also make a stepped A type volume control using relays and resistors, using a microcontroller to control the relays.  There are kits around for this.

Transformer volume control - this type uses a transformer to change the voltage of the signal.  Normally the most expensive, although stepped A's can get pricey too.    This type does not have the impedance issues that resistor types do, it actually has a lower output impedance.  But winding a good transformer is an art, so you should not cheap out here.  intactaudio has a very good reputation

EDIT:
Forgot about "lightspeed" type attenuators.  This is like what Tortuga offers.  It's another way to do a resistance based attenuator.

Randy

glynnw

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Re: Preamp explanation please
« Reply #8 on: 30 Oct 2017, 04:35 pm »
and then there is the LDR type, felt by many to be the best - as built by Tortuga, an AudioCircle sponsor

BigChubby

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Re: Preamp explanation please
« Reply #9 on: 3 Nov 2017, 01:23 pm »
All good information in the above posts.  I would add one additional parameter to consider.
You need to know the "sensitivity" of the power amp you plan to use.  What is the voltage range allowed or needed at the amp input to "drive" the amp to full power.  Some are low - 0 to 2 Volts rms,  some need more - 0 to 7 volts rms.  Note the RMS - some amps and preamps are spec'd as PP or peak to peak.  When comparing preamp out voltage with amp "in" voltage, be sure the units are the same.

Good luck with your project.