5687 preamp

[vintage_dog]

i thought i could complete this over the weekend. anyway, i had most components hooked up on a spare amp, except for a 7k/10w dropping resistor.

here's the diagram i'm using.

http://www.netaxs.com/~vkalia/5687.html

[mozilla]

Hope you can finish it in time for the preamp shootout. Looking forward to making one myself.

[mickey]

I havent started putting mine together !!! So embarrasing. Hope I can get mine up on time. Hey wait a minute I stick have a pack of viagra. no problem...

I'm planning to use a 6N1P for my preamp. keeping my fingers crossed it works. If we have time during the shootout, maybe we can exchange notes on our designs.

[mickey]

Hey! Wait a tick, here is where we get to share notes without waiting for us to get together.

Francis one thing I've seen in preamp designs is that special importance is given to the power supply. some use tube or IC regulators, some load their preamps with high value caps. Saw a site that compared tube regulated vs cap loaded. the cap loaded is still better. Making a preamp suitable for all amps needs special considerations too. I have yet to confirm this but I think input and output impedances play a big part here. designing the volume control stage itself needs some thought. sticking the wiper of a 100K pot directly to a high gain grounded cathode preamp messes up the highs. I have experinced this on a DIY preamp. I hope I was able to confuse you so you wont finish your preamp that soon.

[vintage_dog]

mickey,

thanks for sharing your obeservations particularly the cap loaded vs tube regulated. most preamps i've worked with have been cap loaded (by chance and not by science ).

-- also, i've been curious to really hear the diff between tube rectified and diode rectified power supplies and

-- AC versus DC fillament voltages. foreplay and most vintage preamps are ac on filaments.

francis

[mickey]

If you're a purist by heart then dont go the way of the solid state rectified. some may want to try it which is good since they will know how each would sound like. I may be called lazy or having a one track mind for sticking to tube rectifiers, but like what JM has mentioned, solid state generates odd harmonics. DC heaters are highly recommended. with this you can put a high value potentiometer for a volume control so your source will see a higher output impedance thereby increasing the high frequency cutoff. but this can be applied only for cascodes or similar configurations where the miller capacitance of a common cathode or grounded cathode significantly reduced. if a grounded cathode sees a high input resistance, the high's will be sacrificed.

One of the reasons they use a lower (100K) volume pot is to minimize the hum from an ac heater.

I hope I make sense. I have yet to assemble mine. Hehe

[mickey]

I'm a bit confused, which devices generate odd and even harmonics? JM will kill me for this.

[arnoldc]

tubes generate ODD, i think

[mozilla]

Tubes produce EVEN ordered harmonics.

Music consists of musical tones or notes. A scale in its basic form has eight tones. The sound of an instrument consists of much more than single notes. Each note is a complex collection of a fundamental, overtones and harmonics.

The basic sound characteristic of an instrument is determined by the strength of the first few harmonics. Each of the lower harmonics produces its own characteristic effect when it is dominant or it can modify the effect of another dominant harmonic if it is loud enough. In the simplest classification, the lower harmonics are divided into two groups. The odd harmonics (third and fifth) produce a "stopped" or "covered" sound. The even harmonics (second, fourth, and sixth) produce "choral" or "singing" sounds.

It is generally accepted that tubes are musically superior audio devices because they produce a higher portion of even rather than odd-ordered harmonics. Transistors, on the other hand, tend to emphasize the odd-ordered harmonics. Even-ordered harmonics are musically related to a given frequency in terms of octaves, while odd- ordered harmonics are dissonant. A higher proportion of even-ordered harmonics more closely resemble the natural harmonies in music. A signal passing through a tube will be automatically altered in such a way as to be in sympathy with the normal characteristics of music. Thus, tube amplifiers are commonly called "musical" or "warm" sounding.

The second and third harmonics are the most important. Musically the second is an octave above the fundamental and is almost inaudible; yet it adds body to the sound, making it fuller. The third is termed a musical twelfth. It produces a sound many musicians refer to as "blanketed". Instead of making the tone fuller, a strong third actually makes the tone softer. Adding a fifth to a strong third gives the sound a metallic quality that gets annoying as the music gets louder. A strong second with a strong third tends to open up the "covered" effect. Adding the fourth and the fifth to this gives the sound a more "open" quality.

Higher harmonics, above the seventh, give the tone "edge" or "bite". Therefore too much edge can produce a raspy, slightly out-of-tune quality. Since the ear is very sensitive to the edge harmonics, controlling their volume is very important.

The basic cause of the difference in tube and transistor sound is the weighing of harmonic distortion characteristics. Transistor amplifiers exhibit a strong component of third harmonic distortion producing a "covered" or restricted sound when driven hard. Alternatively a tube amplifier when overloaded generates a whole spectrum of harmonics. Particularly strong are the second, third, fourth, and fifth overtones which give a full-bodied "brassy" quality to the sound. Tubes sound louder and have a better signal-to-noise ratio because of the extra subjective headroom that transistor amplifiers do not have. Tubes get punch from their naturally brassy overload characteristics. The feeling of more bass response is directly related to the strong second and third harmonic components which reinforce the "natural" bass.

[mozilla]

In a comparative review of the ST70 and the ss rectified MkII, it was shown that the ss rectified ST70 has more bass & punch due to the ss rectification.

I also read before that tube rectifiers do not affect the sound as they are not w/in the signal circuitry, BUT after personal experiences in rolling tube rectifiers it is obvious that they do affect the sound.

[vintage_dog]

wow guys. this is one reason this forum is such a big help to audiophiles...sharing of valuable experience, knowledge. and it's all for free

this bulletin board also makes it so easy to catalogue, archive and reference discussion topics.

[arnoldc]

i saw this comment about the 5687 pre-amp in the internet, and i quote "The preamp is burning in (as of August 1, 2002) so I will not describe the sound much except to say the bass sucks, midrange is glaring, and the gain is too high."

[mozilla]

Hope Francis's 5687 preamp will be ready for the preamp or the LS shootouts. Can't wait to hear it sing.

[mickey]

arnold,
Some people say the 300B can't go that low too. Yes some tubes have this limitation but that doesn't mean nothing can be done to correct it(increase the grid leak resistor or the value of the signal cap). Audionote used the 5687 tube as a driver for its 300B, I dont think they would use the tube if it sounds bad. I dont see people using 6922 as drivers for their 2A3 but this I can say, It sounds sooooo goooood.
The problem here is that were still in our eggshells, as we try to make different gears using different tubes and most importantly to share the characters of these tubes with our fellow audiophiles, we then get to break out of our shells. Till that happens, we can only use the comments on the net to guide us where we want to go.

mickey

[arnoldc]

mickey, where, where is that? he he he i'm doing the 6sl7 - 2a3 combo for now but if 6922 - 2a3 combo can be similarly built, that's great!

[yeo]

i saw this comment about the 5687 pre-amp in the internet, and i quote "The preamp is burning in (as of August 1, 2002) so I will not describe the sound much except to say the bass sucks, midrange is glaring, and the gain is too high."

wow! this is definitely not my experience! do you have a link to show this guy's schematic?

mickey is right in saying that something definitely can be done. i think something wasn't done right in that preamp.

yeo

[arnoldc]

yeo, i can't remember the URL. i was just looking for 2A3 stuff when i came across with that page. i read the entire article and the guy has problems all over for a very simple circuit. i remember the schematic to have a 7K 10W resitor in the plate. i think...

I have a 2A3 circuit that uses 5687 tubes as the driver tubes. I looks good but it use too many chokes (plate choke, grid choke, 2 x power chokes, arrgh!) and them chokes are MagneQuest (pricey).

[vintage_dog]

couldn't find the right values of power resistors to give me 7K and 10W for my preamp. I need two of those. pass muna ako sa preamp

[johnmarc0]

After reviewing all your msgs, it seems that everyone needs a little repreive on the preamp shootout sked, so I suggest that on the feb session, will have the shootout and we move the shootout on the second week of feb so as to give everyone time to tidy up their designs.

For me, I will be using a linestage driven by a passive pre non 5687 laid out on NZ pineboard. Anyone who needs the pineboard can have my spares I think I still have 3 to 4 pieces rought sanded and varnished. This pinewood boards are thicker than what ed fausto uses for his darling.

If there will also be enough concensus, we can have a pre session where we can assist people to start building there preamps (so they will not procastinate and we will have enough entries). Any ideas when, where and how this pre session will occur I am open to suggestions. Racio, Ed, Mickey and Edrich always have the pre sessions in my place on weeknights usually wednesdays.

Any ideas?

JM