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300B SET Design Project
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Author:  Suncalc [ 11 Nov 2010, 17:20 ]
Post subject:  Re: 300B SET Design Project

Gio;

I actually considered the use of protection diodes. However, the LT1085 data sheet has this to say on the subject:
Quote:
Protection Diodes

In normal operation, the LT1083 family does not need any protection diodes. Older adjustable regulators required protection diodes between the adjustment pin and the output and from the output to the input to prevent over stressing the die. The internal current paths on the LT1083 adjustment pin are limited by internal resistors. Therefore, even with capacitors on the adjustment pin, no protection diode is needed to ensure device safety under short-circuit conditions.

Diodes between input and output are usually not needed. The internal diode between the input and the output pins of the LT1083 family can handle microsecond surge currents of 50A to 100A. Even with large output capacitances, it is very difficult to get those values of surge currents in normal operations. Only with a high value of output capacitors, such as 1000μF to 5000μF and with the input pin instantaneously shorted to ground, can damage occur. A crowbar circuit at the input of the LT1083 can generate those kinds of currents, and a diode from output to input is then recommended. Normal power supply cycling or even plugging and unplugging in the system will not generate current large enough to do any damage.

Given this information, I decided to leave the protection diodes out of the circuit. As for your suggestion about the capacitors normally being tantalum, in general I would agree. However for the output, I felt that the additional smoothing provided by the larger value capacitor would offset the low ESR of the tantalum capacitor. It might be beneficial however to include a 10µf tantalum in parallel with the 200µf output capacitor. That way we get the best of both worlds; stability and extra smoothing. :up:

Author:  Suncalc [ 12 Nov 2010, 13:46 ]
Post subject:  Re: 300B SET Design Project

As promised, here is the summary post, complete with schematics, for the entire 300B stereo amp.

Specifications:
Peak power out : 7.5W/channel
Input sensitivity for peak output : 5.1Vpeak (3.6Vrms)
Frequency response (-3dB BW) : 20Hz to >20KHz
Distortion at full output : 3.3%
Distortion per watt (Ds) : 0.44%/W
Output impedance : 8Ω
Input impedance : 200kΩ + 69µµf

Amplifier Schematic (2 channel)
Attachment:
Schematic Final Amp.png

Power Supply Schematics (120v and 240v mains versions)
Attachment:
Schematics PS final.png

300B Filament Supply Schematic
Attachment:
Schematic Filament Final 240.png


Thermal Design Note:
The bridge rectifiers need to be heat sinked to a heat sink with a thermal resistance of no more than 39.5˚C/W (including interface layer thermal resistance). I recommend the Comair Rotron 822202B00000 heat-sink with a thermal resistance = 13˚C/W (http://www.alliedelec.com/search/productdetail.aspx?SKU=5990351). The LT1085 rectifiers need to be heat sinked to a heat sink with a thermal resistance of no more than 11.8˚C/W (including interface layer thermal resistance). I recommend the Aavid Thermalloy 529802B02500G heat-sink with a thermal resistance = 3.7˚C/W (http://www.alliedelec.com/search/productdetail.aspx?SKU=6190109). The thermal design is based on a peak ambient temperature of 75˚C at the heat sink. As such, the case containing the filament supplies should be vented but forced air cooling should not be required.

Now I'll be handing this thread off to Mark so he can keep us up to date as the amplifier comes together. I'm really looking forward to seeing how this comes out. :headphones:

Author:  mwhouston [ 12 Nov 2010, 17:44 ]
Post subject:  Re: 300B SET Design Project

Matt:

I think you wanted to know the DC resistance of the 193M choke. It is a low 60ohms.

The Hammond OPTs, choke and the wrong filament transformer arrived today. In all 30lb of iron. I now have a double hernia after carrying it box to the car..

The OPTs are bigger then the biggest power tranni I have ever used the choke is so big just standing near it has reduced my tinitus!!

Author:  Suncalc [ 12 Nov 2010, 18:18 ]
Post subject:  Re: 300B SET Design Project

Cool! The big iron has arrived! :thumbsup:

The 3Ω delta on the choke shouldn't be an issue.

I would like to know the DC resistance on the primary side of the output transformers (to check the 200Ω assumption I used in the design). I would also like to know the DC resistances of the primary and HV secondary of the power transformer. These values affect the source resistance going into the smoothing filter and hence the DC voltage out. If I have all of these numbers then I can check the power supply design and confirm (or recalculate) the value of the voltage dropping resistor (currently 150Ω 6W in the 240v mains power supply).

Funny you should mention a "double hernia".. I've been at home for the last three days on muscle relaxers and steroids after I threw out my back last weekend. This week I've been asking my wife to move the heavy stuff.

If you get a chance post some pictures of the iron. I'm interested to see how big it all is. Maybe a separate power supply chassis for this amp in a good idea. :lightbuilb: Didn't you mention that you were thinking about going that way?

Author:  mwhouston [ 12 Nov 2010, 20:49 ]
Post subject:  Re: 300B SET Design Project

I was just zapped by the primary of the Hannond OPT. I use an analogue multi-meter. While measuring the primary DC resistance of the Hammond OPT I was holding the meter probes on with my fingers. Of course as I let go the back EMF got me. It was a fair belt. This has to be the first time I have got a shock from something not even plugged in!! Ouch..

Matt: DC resistance of the OPTs is 120ohms. The power tranni is still being wound.
Attachment:
OPTss.jpg

Author:  Gio [ 13 Nov 2010, 12:27 ]
Post subject:  Re: 300B SET Design Project

Ouch. :hot: Remember that a multi-meter puts out about 2 to 3 V when measuring resistance. This could have something to do with getting zapped.
Suncalc wrote:
As promised, here is the summary post, complete with schematics, for the entire 300B stereo amp.

Great. I added a link to the summary post from the first post. Perhaps when this is all together we can consoidate it into a single page on the main site.
Cheers

Author:  Suncalc [ 13 Nov 2010, 20:32 ]
Post subject:  Re: 300B SET Design Project

Oops, I found another one. :blush: On the filament schematic posted above, I said that the mains supply was 240 (234) but I wired it like it was a 120v main. So here is the corrected schematic with the proper primary side wiring.
Attachment:
Schematic Filament Final 240.png

I hope that it's a simple enough mistake that most people will catch it.

And to keep one other promise which I made. I am going to include here the alternate schematic for the AC 300B filament heaters. When the 300B was introduced, the state of music reproduction was somewhat different then it is today. The 300B was introduced in the late 1930s. At that time it was accepted that there would be some slight hum in the background of most amplifiers. The thought was that the hum/noise level will be many dB down from the audio so it will be hidden in the music. For this reason, the 300B was introduced as a DHT. It really wasn't until later that the unipotential cathodes became standard on most audio tubes.

In the post on the filament supply I calculated the unbalanced ripple level on the cathode of 2.2mV. Frankly getting this level of balance using a "hum pot" is going to be nearly impossible. This is the equivalent of getting a single turn (270˚) pot adjusted to within 0.12˚. If we go to a ten turn potentiometer (3600˚) then this 2.2mV requires getting that adjustment to within about 1.6˚ (still very difficult). So I decided that I'd specify a ten turn pot. On this potentiometer, getting within 16˚ will balance the ripple to about 22mV (-70dBv). So dependent on the skill of the builder, the hum should be able to be cancelled to somewhere between -70dB and -90dB. The circuit is obviously much simpler, but the sonic results will not be quite as good.
Attachment:
Schematic Alternate Heater.png

In theory you could put a single turn potentiometer between two 100Ω 1% resistors and get essentially the same result as the 10 turn potentiometer, but I decided to minimize parts count (That and the 10 turn pot is only $10.25 USD at Allied Electronics Supply).

Gio; Could you put a note on the summary post about the mains wiring mixup? Thanks. ;) - Changes made - 15 Nov 2010 :cop:

Author:  sampleaccurate [ 17 Nov 2010, 23:55 ]
Post subject:  Re: 300B SET Design Project

May I make a suggestion? Use higher power cathode resistors. The power dissipation of over 6 watts is going to make those 5 watt resistors extremely hot even with two 5 watt resistors in series. I used two 20 watt resistors in parallel (40 watts total capacity) and they still get very hot.

Just my 2 cents based on experience with essentially the same amp.

Nice design BTW. Very impressive.

Author:  mwhouston [ 18 Nov 2010, 13:02 ]
Post subject:  Re: 300B SET Design Project

Matt: What about not using regulators on the 300B heaters. What about just reticiation with good filtering. I can see the problem the regulators have with the differential between the output voltrage and the input voltage being very small. This is not why I don't want to use regs, I'm wondering can we obtain good quiet DC heater current without regulation. It does reduce complexity.

Author:  Suncalc [ 18 Nov 2010, 20:08 ]
Post subject:  Re: 300B SET Design Project

Quote:
May I make a suggestion? Use higher power cathode resistors. The power dissipation of over 6 watts is going to make those 5 watt resistors extremely hot even with two 5 watt resistors in series. I used two 20 watt resistors in parallel (40 watts total capacity) and they still get very hot.

I understand your concern. I chose the 25 series Ohmite resistors because of their excellent derating factor. These resistors derate from 100% to 0% from +25˚C to +350˚C. Because all the thermal design for the amp is based on a max internal temperature of +75˚C, these 5W rated resistors actually derate to 4.23W at the max temperature. Their actual dissipation is ~3.16W (0.095A^2 * 350Ω = 3.16W) so this represents a design margin of (4.23-3.16)/4.23*100% = 25.3%. I understand that they may get warm in this configuration but they are still well within their ratings. And, since they are contained on the inside of the chassis, I didn't think this would be an issue. If we really wanted to keep these resistors cool, I would recommend going to the Ohmite 825J series which have the screw tabs for mounting to a piece of metal as a heat sink.

Quote:
What about not using regulators on the 300B heaters. What about just reticiation with good filtering. I can see the problem the regulators have with the differential between the output voltage and the input voltage being very small. This is not why I don't want to use regs, I'm wondering can we obtain good quiet DC heater current without regulation. It does reduce complexity.

I agree with reducing complexity and the totally passive approach was my first idea. The problem I encountered was that because of the very small source resistance of the supply, I couldn't find a way to get anywhere near the target ripple level of 2.2mV. Using the 6.3V transformer the total drop voltage we can see in a filter is 6*sqrt(2)-1-5 = 2.48v. The source resistance is something very small (~.25Ω) for the winding plus 1V/1.2A=0.83Ω for the diode bridge for a total around 1.1Ω. Even with the current 6800µf primary filter cap, the input ripple to the regulator is still on the order of 1v. Theoretically, to get it down to the target ripple we could use a 2Ω series dropping resistor but we would need a ~0.4F capacitor (Or we could use four 0.25Ω resistors and four 100,000µF capacitors). :eek: You begin to see my problem? In the interest of trying to reduce complexity the filters are getting very complex. After some consideration, I thought that the low dropout linear regulator was the best option.

There is a possibility of using a combination of the hum-pot configuration with a filtered DC filament supply. In this instance we could assume 70dB of filtering by the hum-pot and get the additional filtering via passive filtering on the DC filament supply. In this case the ripple on the DC line would only have to be reduced by -20dBv to approximately 1/2v. The cathode resistors would need to be reduced to offset the additional 2.5V bias voltage, but that should be manageable. What do you think? Of course the other option is to go with the AC heater/Hum-pot configuration and live with the -70dB hum level as opposed to the -90dB target value.

Anyone else have any thoughts on this subject?

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