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Old 12th April 2010, 10:14 AM   #1
ervinl is offline ervinl  Indonesia
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Default New bie: some questions, want to start DIY pre amp

Hi,

I would like to start DIY pre amp tube. There are some topologies, CCS, conventional R and SRPP. Which one will be better, have less hums/noise for pre-amp?

About filament, some said it is better to straight connect to AC of transformator. If I do like this, will it be potential to hum or not, since it does not relate to signal, and I found classical tube amp doesn't rectify the filament supply. But another people even use so-regulated for this, e.g. not just rectifier, but also big cap and big regulator e.g. LT1083 or MOSFET. I don't see any benefit of this.

About hum, since this is pre amp (based on SS experience), hum is more likely to exists in higher gain/low signal circuit like pre amp. Then, how to avoid/minimize hum in tube pre amp? Usually in SS, I use regulated PS e.g. using 78XX/79XX IC, but since this is a tube, I can not do like that.

Will using tube rectifier reduce hum possibility than dioda (even fast rectifier like SF, UF or MUR)?

What value of capacitor minimum in PSU, does simple bridge + C enough?

I plan to use regular (EL) transformer, may be not toroid.

Thanks,

Ervin L
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Old 12th April 2010, 07:15 PM   #2
kmtang is offline kmtang  Canada
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It is fun being a DIYer that you can experiment various configuration of the amplifier such as CCS, conventional R, choke plate load, LED or battery cathode biasing etc etc.

Using tube rectifier for the filament is just not practical at all. People say run the filament with CCS would sound good as AC supply but without hum.

Take your soldering iron and start building it.


Johnny
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Old 13th April 2010, 03:47 AM   #3
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You can run a 317 series regulator for HT, just float it! Just watch the 37 Volt input/output limitations. Study some three pin HT regulator schematics....it will all fall into place when you understand how the circuit works.

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Old 13th April 2010, 04:29 AM   #4
chrish is offline chrish  Australia
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Or use a pass device, maida regulator Maida regulator
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Old 13th April 2010, 05:56 AM   #5
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by ervinl View Post
I would like to start DIY pre amp tube. There are some topologies, CCS, conventional R and SRPP. Which one will be better, have less hums/noise for pre-amp?
Many choices. Depends on the gain you need and your taste. The TubeCad site is a good source of info on various topologies.

Quote:
Originally Posted by ervinl View Post
About filament, some said it is better to straight connect to AC of transformator. If I do like this, will it be potential to hum or not, since it does not relate to signal, and I found classical tube amp doesn't rectify the filament supply. But another people even use so-regulated for this, e.g. not just rectifier, but also big cap and big regulator e.g. LT1083 or MOSFET.
Unless you are building a very high gain preamp, such as a phono amp, and unless you are using tubes with direct heated filaments (DHT's), a DC filament supply is unnecessary. Indirect heated tubes can be very quiet with AC heating. That's a lot of what they were designed for. Good layout is important, and lifting the filament reference 20V or so above common is good practice.

Quote:
Originally Posted by ervinl View Post
Will using tube rectifier reduce hum possibility than dioda (even fast rectifier like SF, UF or MUR)?

What value of capacitor minimum in PSU, does simple bridge + C enough?

I plan to use regular (EL) transformer, may be not toroid.
Both tube rectifiers and silicone diodes can make quiet HV supplies. A matter of taste.

A regulated supply can be used, and some prefer the sound of specific regulators. However, regulated B+ is not necessary for a quiet preamp. Again, a matter of taste. But, a quiet supply is always good, so you will need decent filtering either way. More if it's not followed by a regulator.

Download PSUD from the Duncan Amps site. It's free and works very well for designing passive supplies.

Sheldon
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Old 14th April 2010, 01:22 AM   #6
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I would start with a simple common cathode gain stage with a well filtered B+ (maybe CLCRC) and AC heaters. Then you can add fancy stuff later and see for yourself. Use a low or medium mu triode like 6SN7, 6N1P or 12AU7 depending on what you have access to. Give it a relatively high B+ voltage and plate load for the particular tube in question. Lay out the heaters carefully with twisted leads and elevate the heaters above ground by say 40V or so to reduce hum. If you have to drive a SS power amp or long cables consider using a cathode follower stage on the output.

You can then experiment with the cathode by trying unbypassed cathode bias, bypassed cathode bias, LED bias and so on. Then play with the plate trying choke loads and various active loads. Who knows you might find nirvana in a preamp with more active devices than the Space Shuttle... but you may prefer the simpler approach. I say do the easy stuff first.

If your PS is well filtered and you have a decent load on the plate of the gain stage you may find no need for things like CCSs, mu followers etc.

As for tube or SS rectifiers your choice. Tube is more expensive but they are simple.
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Old 14th April 2010, 02:30 AM   #7
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A "gotcha" with common cathode line stages is trouble with low impedance loads, like those some SS power amps present. Few tubes, in common cathode, can directly drive the IHF 10 KOhm "standard" load. A happy exception to that state of affairs is the 12B4. Plenty of 12B4 designs are available. I've uploaded my version.

The power supply shown uses a pseudo choke I/P filter. Choke I/P filters are inherently well regulated, unlike the cap. I/P filters you may be familiar with.

It seems you are in a "240" V. power mains zone. Both a Triad N-68X, for B+, and a Triad VPL24-400, for heater power, have dual primaries. Therefore, that "iron" works in both "120" and "240" V. zones and it's reasonably priced.
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Old 14th April 2010, 03:41 PM   #8
palmas is offline palmas  Portugal
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First tell me what preamp you need:
Gain
input impedance
output impedance
max voltage swing

An then we can discuss what might work well.
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Old 14th April 2010, 05:20 PM   #9
jeepy is offline jeepy  Switzerland
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Quote:
Originally Posted by ervinl View Post
Hi,

I would like to start DIY pre amp tube. There are some topologies, CCS, conventional R and SRPP. Which one will be better, have less hums/noise for pre-amp?

I built a simple one based on the Eudidice (easy to find on the web) and it was Ok but I felt that I had enough tubes with my amplifiers and I built a solid state Kaneda DC which is very good with an incredible bandwith and no capacator. However since the output level of most of the sources is enough to saturate the amps I am know using a passive preamp wich is great with a 100 K Alps blackbeauty pot + one 10 K Caddock resistor on both sides of the cursor for each channel and two amorphous Lundhal small line transformers to adapt the impedance This is the key point. If you do need gain (only) this is the best and easy solution. Just adapt correctly the impedance.


What value of capacitor minimum in PSU, does simple bridge + C enough?

I plan to use regular (EL) transformer, may be not toroid.

Thanks,

Ervin L
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Old 14th April 2010, 05:26 PM   #10
jeepy is offline jeepy  Switzerland
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I am sorry but I do not know why my last post was inserted in the message of Ervinl!
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