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Old 22nd April 2015, 02:07 PM   #1
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Default Preamp needed?

It has been a long while since I've been on the forums or had any time to do any audio tinkering...the ole "real life got in the way" excuse... However, I have recently set up a headless Raspberry Pi-based RuneAudio MPD system for my digital music to sit alongside my turntable in my living room. I planned on running the output of the little USB DAC that I have attached to the Pi to my tube power amp (~25W with KT88-power tubes built from an excellent kit made by forum member scott17). However, it seems that I have to crank the power amp to ~3/4 volume to get any sort of reasonable listening volume (on the low end of reasonable). As such, I think I might need a preamp. I don't want to listen at ear bleeding volumes, but I don't want to have to crank the power amp to be able to hear anything.

As such, I think I'm in need of a preamp. I am, however, not particularly knowledgeable as to what would be a good pairing with my power amp. My questions are as follows:

1) How much gain do you think I would actually need?
2) Tube or Solid State? (Would building a tube preamp result and coupling with a tube power amp result in a little too much "tube character" imparted on the source material? Would a solid state pre be a better choice? )
3) Based on your opinions for questions 1) and 2), do you have any recommendations? Kits and non-kit schematic recommendations are welcomed!

Thanks!
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Old 22nd April 2015, 04:52 PM   #2
AndrewT is offline AndrewT  Scotland
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and what does it sound like when you "crank it" round to full on?
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Old 22nd April 2015, 05:46 PM   #3
DF96 is offline DF96  England
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Quote:
Originally Posted by BlackEdition
I don't want to have to crank the power amp to be able to hear anything.
What do you suppose happens around full volume, apart from the best possible signal/noise ratio and (probably) the flattest frequency response?
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Old 22nd April 2015, 05:53 PM   #4
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@AndrewT and @DF96

It sounds fine, it just seems precarious to be running the power amp at full tilt for tube longevity reasons. I could be off base.
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Old 22nd April 2015, 06:03 PM   #5
DF96 is offline DF96  England
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Why precarious?

Unless there is something very unusual about your setup then running at a particular volume level has a certain effect on valve life. Running at the same volume via a preamp and extra attenuation (e.g. turn down the volume pot, to throw away the gain you just introduced) has exactly the same effect on valve life. There are two differences:
1. you pay for more electricity as you have an extra box to power
2. you wear out a different part of the volume pot track

Most people have too much gain. It sounds like you have just the right amount!
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Old 24th April 2015, 04:20 AM   #6
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If you don't have to deal with impedance matching on your tube amp input then you might be interested in either of these Behringer units. Both go for $20-30 on eBay/Amazon. eBay is a better bet for free shipping.

Microamp HA400 is billed as a headphone distribution amp, but its Zin of 100k, and Zout of 80 ohms, make it suitable as a preamp-distribution amp.

Microphono PP400 is a single-purpose phono preamp with RIAA equalization.

Or, of course, you could build something, such as the below. But at $25 for either of the Behringers (including power supply) you gotta wonder whether it's worth waiting for the iron to heat up.

Quote:
Originally Posted by BlackEdition View Post
1) How much gain do you think I would actually need?
Not much. If your amp has a gain of 20, then a gain of 3 in front of it would make a total gain of 60 available. Two stages of gain adds up--or rather multiplies--fast.

Quote:
Originally Posted by BlackEdition View Post
2) Tube or Solid State? (Would building a tube preamp result and coupling with a tube power amp result in a little too much "tube character" imparted on the source material? Would a solid state pre be a better choice? )
These things are so very much in the ear of the beholder. Your final stage is tubes, so I personally vote that few if any would hear a difference in preamps.
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Last edited by bentsnake; 24th April 2015 at 04:26 AM.
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