Tube vs. Solid State Preamp
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From: Adrian Tuddenham on 31 Mar 2010 05:50 Paul Keinanen <keinanen(a)sci.fi> wrote: > On Tue, 30 Mar 2010 23:47:40 +0100, > adrian(a)poppyrecords.invalid.invalid (Adrian Tuddenham) wrote: > > >The circuit design is critically important, especially the method of > >matching the input impedance to the source impedance. If you throw away > >signal at that point, no first stage valve or transistor, however > >exotic, will achieve the best S/N ratio. > > In some direct conversion receivers with the SBL-1 diode ring mixer, > quite interesting audio preamplifiers are used. For best performance, > the mixer "IF" (in this case audio) port had to be terminated into a > 50 ohm load. Often a common base amplifier stage was used, sometimes > consisting of multiple small signal transistors in parallel or using a > big power transistor for best noise performance. > > Perhaps such amplifiers could be used with low impedance microphones > as well. Impedances between a few ohms and a few tens of ohms are common in moving coil stereo record pickups. The common-emitter configuration is often used in their pre-amps, with many paralleled large-area switching transistors (the ZTX series being particularly popular). <http://sound.westhost.com/p25_fig1.gif> [Doug Self's design] > The microphone characteristics, such as frequency response is usually > specified for some standard load impedance, loading the microphone for > power match may alter these characteristics, so some corrections may > be needed in the following stages. That is true, although the difference with modern designs is usually quite small. Some older designs relied heavily on the correct electronic damping. By using two independent feedback paths it is possible to make the termination appear different from that of the active device alone. One path drives voltage into the reference terminal of the first stage to set the voltage gain; this raises the apparent input impedance above that of the active device. Another path drives current into the input terminal, which lowers the apparent input impedance - and because the current is supplied through a feedback loop, the noise of the resistors in that pathway is much reduced by the effect of feedback. The two feedback signals need to be in opposite polarity, this can be achieved with a simple inversion stage as in the circuit below: <http://www.poppyrecords.co.uk/other/images/PreampAttenuator2.gif> Care has to be taken that the extra phase shift of the inverter does not cause loop instability. Alternatively both signals can be of the same polarity but the voltage feedback can be applied to the 'earthy' end of the input transformer or the input device. This means that several parameters of the transformer or input cabling which are often ignored, must be tightly specified - in particular capacitance between the 'cold' end and earth. If a transformer is used in the output stage, it is much easier to derive two different polarities of feedback. <http://www.poppyrecords.co.uk/other/images/AMC5.pdf> This elegant BBC circuit does that by running a floating feedback winding through resistors to earth at both ends (R11 & R16) - and manages to include stepped gain as well: -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
From: John Larkin on 31 Mar 2010 11:29 On Wed, 31 Mar 2010 13:36:54 -0800, Robert Baer <robertbaer(a)localnet.com> wrote: >John Larkin wrote: >> On Tue, 30 Mar 2010 08:56:08 -0700 (PDT), Chris >> <christopher.maness(a)gmail.com> wrote: >> >>> With today's modern technology, is it possible to make a solid state >>> preamp that is as quiet as a good tube pre? >>> >>> I am thinking about building a preamp. >>> >>> Thanks, >>> Chris >> >> Tubes are noisy. >> >> John >> > Try running a 12AU7 with a plate supply of 45V and grid-to-cathode >voltage of about -1.5V (tweaked for zero grid current). What are the voltage and current noise densities? John
From: Chris on 31 Mar 2010 12:25 On Mar 31, 2:45 pm, Robert Baer <robertb...(a)localnet.com> wrote: > Jim Thompson wrote: > > On Tue, 30 Mar 2010 10:41:07 -0700, Joerg <inva...(a)invalid.invalid> > > wrote: > > >> Jim Thompson wrote: > >>> On Tue, 30 Mar 2010 09:49:03 -0700, Joerg <inva...(a)invalid.invalid> > >>> wrote: > > >>>> John Larkin wrote: > >>>>> On Tue, 30 Mar 2010 08:56:08 -0700 (PDT), Chris > >>>>> <christopher.man...(a)gmail.com> wrote: > > >>>>>> With today's modern technology, is it possible to make a solid state > >>>>>> preamp that is as quiet as a good tube pre? > > >>>> Sure, but it will be shunned by tube-freaks :-) > > >>>>>> I am thinking about building a preamp. > > >>>>>> Thanks, > >>>>>> Chris > >>>>> Tubes are noisy. > > >>>> Not at all. I remember when I was young and those super low noise RF > >>>> FETs came out. Everyone (who had the dough to buy those) jumped on them, > >>>> only to find out that the old nuvistor preamp was in about the same > >>>> ballpark noisewise but had a dynamic range from here to the Klondike > >>>> while them thar newfangled trainsistahs didn't. > >>> If you know how to bias BJT's for low noise, you can make phenomenally > >>> low audio noise preamp's. > > >>> Sheeesh! Back when I was a kid I could make a reasonably low noise > >>> preamp with Ge devices... low current and low VCE. > > >> When I was a kid that wasn't needed. The "lowest noise" audio source > >> were 45rpm records and even that didn't matter. Because in our time it > >> was all rock music. The louder the better. So the race was on who could > >> build the biggest honking amplifier. My limit was reached when the > >> breaker on a typical European 230V/16A circuit would trip off upon a > >> heavy-handed twang on the electric guitar :-) > > > You're still a kid :-) > > > My record for a solid-state amplifier is 400W (RMS, sustained, fan on > > a heat-sink "tunnel" :-) > > > ...Jim Thompson > > A lot better than my record for a tube amp at 100W RMS sustained, > with total hum and noise under -60dBmW. > Do not remember how far down it was, but cranked up all the way it > made no difference open input or shorted input, one could not hear noise > and/or hum with ear right at speaker. Do you still have the schematic of your amp? Also, did it have a mic level pre? or just a line level input? Chris
From: Chris on 31 Mar 2010 12:30 Also, did anyone get a chance to listen to the mp3 I posted? I thought the end result was pretty nice. A cheap tube pre (I would bought a SS if they sold any), and a ~$100 PCM recorder. Chris
From: Michael A. Terrell on 31 Mar 2010 13:04
Joerg wrote: > > The German text in there is rather funny. For "low noise" they use the > expression "niedriges Geraeusch". That words normally describes a noise > such as the squeal of a bearing that's about to go bad. Or heavy metal. ;-) -- Lead free solder is Belgium's version of 'Hold my beer and watch this!' |