Building Started...

[rlyach]

Paul,

The percent error calculation I used is exactly the same as what you showed. The db error that I calculated is the db difference between the channels with respect to the intended db attenuation. I simply took the intended attenuation and multiplied by the percent error. The way I did it may not be the correct way to look at the db error. Your formula shows the db difference between channel A and channel B, which seems simpler and more straight forward. Here is another plot of db mismatch plotted against the average db attenuation rather than resistance. It shows that the BH supplied POT is better than the RS pot. Unfortunately I did not take more high resistance readings on the RS pot so I can't see its performance at very low volumes. I will repeat this study once I get my Goldpoint attenuator, and I will make sure I add enough data to fill the entire range.

Thanks for the help. I am not an audio engineer so I am still making sense of all this. Most of my experience is with integrated circuit design and manufacturing. I don't mind in the least that you are pushing me. I love learning and find amplifier circuits fascinating. Thanks for the help.

 

[rlyach]

Now I get to be embarrassed. After looking at my data I realized that audio pots are NOT symmetrical.... I confused R1 and R2. Here is the comparison again using the proper POT orientation. Now it is very clear that the POT shipped with my kit is better than the POT that I purchased at Radio Shack. Also, the standard POT only has a max mismatch error of .65db, and only at very low levels. This has been very educational.

Update: The graph has been updated with new and complete data.

 

[rlyach]

Well... I am waiting for my attenuator and switch to come in so I have had to stop. Here is my progress so far. This kit is really fun to build. The manual instructions are very easy to follow. I love the high resolution pictures. You can zoom way in to look for more detail.

 

[Doc B]

Looks good!

 

[jimiclow]

Randy, are you sure you don't want to go the 45 route?;D You can still put a 2A3 in a 45 amp.

 

[rlyach]

Jimiclow,

At the risk of displaying my ignorance and starting another long discussion, I will say that I chose to wire for the 2A3 because of power. I will be driving a pair of Orca  speakers and wanted the extra 2 Watts or so. I did look at the bias point of the 45 setup and it was a little confusing to me. I measured the DC resistance of the plate choke red wire to the black wire. It read ~260 ohms. Based on my calculations, that puts the bias point at 57V on the cathode and 35mA of plate current. Assuming the same ~4K load from the output transformer I calculate a max of 2.5 Watts at 4.9% distortion. I have already purchased a pair of JJ 2A3-40's (match) just for this amp, so I don't think I will switch to 45's.

Do the 45's really sound better. From a purely design perspective (from an admitted novice), the specs seem the same or slightly lower at a lower power output. Also, If I wire for the 45 and put 2A3's in the amp, the bias current will be only ~35mA and the usable range of the plate voltage will drop by about 50V. It will also reduce the output power as compared to the 2A3 wiring.

Now I am waiting for the someone with more experience to set me straight. 

 

[Paul Birkeland]

Assuming the same ~4K load from the output transformer I calculate a max of 2.5 Watts at 4.9% distortion.

Now compare the sound pressure levels available from your Orcas with 2.5 watts vs. 3.5 watts.

 

[rlyach]

That would be an interesting test, but Blumenstein Audio does not publish any sound pressure vs power data. They conservatively spec the Orca sensitivity at 89dB at 1Watt and 1Meter. Just for clarity, I calculated the 2A3 maximum power output at 4.4 Watts as compared to the calculated 2.5 Watts for the 45s. These calculations were based on the published plate curves for the respective tubes. Also, this number uses the full swing of the tube which includes 0V bias, and that is not a good place to operate, so it is better to compare the output power using the specs published for the two different tubes used in the Stereomour, which are 1.75 Watts vs 3.5 Watts. Those are the numbers that drove my decision.

 

[Paul Birkeland]

You can calculate the difference in SPL without knowing the efficiency of the speaker. 

 

[Paul Joppa]

...and it's close to 3dB, which is audible but - shall we say? - not huge. I'd go with the 2A3 myself for Orcas.

Yes, by the way, the available power is in theory a bit greater than what we quote, or what is quoted in the spec sheets. Chalk it up to the transformer losses, about 10% (0.5dB).

Also, the RCA standard (250v, 60mA, 2500 ohm load) is 3.5 watts and I don't want to quote a higher number, people will just think I'm exaggerating. But in fact, at 300v/50mA into 4000 ohms, the theoretical efficiency is around 20% greater - very close to your 4.4 watts in fact.

For what it's worth, there are two "standards" for power. One is the power at clipping, defined as a barely visible flat top to the sine wave output, the other usually as 5% THD. Neither is all that precise, since with no feedback the linearity of the individual tube has a large effect.

 

[Grainger49]

  .  .  .    Blumenstein Audio does not publish any sound pressure vs power data. They conservatively spec the Orca sensitivity at 89dB at 1Watt and 1Meter.  .  .  .   

I'm running my Orcas on "3.5 Watt" (quotes because of Paul) Paramours.  I'm in front of them now, my RS meter says they are playing at the mid 80sdBA.  Much louder than I want to listen.  I do think that Blumenstein is conservative.  Maybe more like 83-85dBW.  In the 90dBA range OSHA requires ear protection.  I require no one to wear ear plugs in the listening room.

 

[rlyach]

Paul,

Thanks for all the information. I feel better about deciding to use 2A3's. If you actually get closer to 4 Watts out of the 2A3's then the SPL difference is actually closer to 4db instead of 3  .  Incidentally, While I was calculating my load lines for the respective tubes I think I came across another advantage of the parafeed design. The musical impedance that the tube sees is a sum of the resistance of output transformer, the reactance of the output transformer and the reactance of the parafeed capacitor. Since the reactance of the cap is 1/(2*pi*f*C) and the reactance of the output transformer is (2*pi*f*L), the parafeed cap's impedance will decrease with frequency and the output transformer impedance will increase with frequency. These seem to act together to give a more stable tube load line over frequency. Maybe I am over analyzing all this. I will use my LCR meter on the various components tonight and see if the numbers add up.

Grainger,

Thanks for the info on the Orcas. I assume the RS meter is a Radio Shack sound pressure meter. How far was the meter away from the speakers when you made the measurement? This might give us some idea as to the effeciency of the Orca's in a listening environment.

 

[Grainger49]

Randy,

Yes, a vintage Radio Shack analog SPL meter.  It was about 12' from the speakers.  Let's see if I have a hosted image of the Orcas from my listening La-Z-Boy?

Yup:

Short legs and short fat (wide) feet.  My ears are at least 12' from the Orcas and as you see they are way away from any reflecting surfaces.

 

[rlyach]

Wow... That calculates to 95 db at 1 meter. If we knew the power level you were listening to we could actually calculate efficiency. We could roughly get that number if we knew the attenuation setting and the max power of the amp. Very nice system and listening setup by the way...

 

[Doc B]

You can't do that kind of 1 meter vs. 4 meters calculation based upon an in-room measurement. You have to take the driver outside or into an anechoic chamber to get values that you can rely upon. In room readings will be higher at a distance than anechoic measurements. PJ can give you the rundown regarding the reverberant field, etc. in remarkable detail. Of course this is a good thing when you are using low power amps and medium sensitivity speakers.

 

[Paul Joppa]

Randy, you are quite right about the load impedance. A well-chosen parafeed capacitor/plate choke combination keeps the load impedance seen by the tube close to resistive, to a much lower frequency than would be the case with a series feed transformer of the same inductance. This is in fact how I arrived at the rule of thumb for parafeed capacitance (2*L/(R*R)). It permits greater power and lower distortion in the deepest bass. Of course, it also assumes a resistive speaker impedance at those frequencies - not always an accurate assumption!

 

[rlyach]

My Goldpoint attenuator came in so I can continue my build. Unfortunately my Orcas will not be here for at least another week, so I won't be able to enjoy the system until then. Here is the final plot of attenuator data. The Goldpoint has virtually ZERO tracking error. In fact the resistors matched so well that the largest mismatch error was only .004db.

 

[drewh1]

Hi Randy - I am waiting for my kit and have appreciated reading about your progress.

Do you mind telling me what value you ordered for the Goldpoint Stepped attenuator?  I would like to get one and some knobs but am not such a newbie to this, I am not sure how to calculate the startgin value.

Thanks,

drew.

 

[rlyach]

Drew,

I just ordered the same value that the kit shipped with, 100K. I also ordered the smallest (1.1 inch) knobs. I think they look great. By the way, I finished my build today and everything works. I only had a pair of cheap RCA 5" two way speakers to play it through until my new Orca's arrive, but even with these inefficient $50 speakers, it sounded amazing. I will post a picture of the top side once my Orca's and the base arrive from Blumenstein Audio. Here is the underside. You can see some of the upgrades, including caps, resistors, the volume pot, and selection switch. I could not be happier with this amp. One thing that surprised me was my hum adjustment actually read 0.0mV on both channels. No hum at all.

 

[drewh1]

Nice Job, looks great and nice to hear it worked the first time. - I'll likely get the GP before I get the kit but wait on Cap upgrades. I want to see If I can actually hear a difference in the caps after I have listened for awhile.

I also appreciate hearing that it sounded good with your cheapie speakers, I have no worries this amp will drive my Green Mountain's adequately.

What do you have planned next?