Looking for first SS amp idea, have PS

Hello, This is my first post to this forum. I've built a number of tube amps from scratch so I'm not a total newby but don't have any SS experiance. I already have a chassis and 36VDC power supply from a Oregon Audionics stereo amp. The power trans is a Western Transformers wti#1820/code#977 into a bridge rectifier and then to two Sangamo 15,000mfd at 60VDC caps. The power trans and the caps are fine and that's where I'm getting the 36VDC. Well, each cap has 18VDC across it. The heat sink has 4/channel Motorola 2N5630. The amp looks like a pro sound amp and I can't find any info on it except that the company's founder now works for Fosgate. Can anyone direct me to a proven DIY design that could make use of this PS and heatsink? I've been looking at the Krell thread and while I know that PS won't work for that amp I'm hoping that someone has board available for a similar amp project. All my speakers have fairly high eff so I don't need that much power.
Thanks in advance for any help, this is a very cool site, John.....
 
Stocker, Thanks. I think I might look into the DoZ also. I think my donor amp was about 100w/ch so it should have enough guts for 10watt/ch class A. Does anyone reading this know if DoZ will work at 36VDC? I really don't need another 75-125watt/ch SS anp as I have two I like now. A little class A sounds like it might be fun.
John..........
 

Greg Erskine

Member
Paid Member
2002-01-05 11:56 pm
Sydney
whatsnext said:
Stocker, Thanks. I think I might look into the DoZ also. I think my donor amp was about 100w/ch so it should have enough guts for 10watt/ch class A. Does anyone reading this know if DoZ will work at 36VDC? I really don't need another 75-125watt/ch SS anp as I have two I like now. A little class A sounds like it might be fun.
John..........

Hi whatsnext.

I run mine at around 32 volts (~24 Vac) and that seems OK. Rod's schematics show 35VDC. There were some lengthy threads about the DOZ about six months ago. Do a search. There were a few modifications to ensure reliablilty. A little class A sounds good and feels hot.

Rod's recommends a CRC PSU, so you will probably drop a volt or 2 across R if you mod your PSU a little.

Good luck
 
Hi whatsnext,

I'll second Greg in trying out the DoZ. It is a great little amp that will run perfectly well at 36V. This weekend I just lowered the voltage from my DoZ from 40V to 30V and although the output is reduced by 50%, in combination with my Buschhorns (Fostex Fe108ES rated at max 8W RMS) there is no subjective difference. Might be even a little more detailed because of more careful matching of the bias from both channels. Even though the dissipated power is down from 136W to 'only' 78W it still runs hot. Just takes a little longer to become too hot to touch... Did not do any modifications to limit current rush since I am a little bit afraid of the power-on thump that results from the mod in combination with the Fostex drivers.
 
whatsnext,

Of course there is a reason why I downscaled to a lower voltage, the most important being that mr. Elliott recommends not to use 40V anymore but keep a maximum of 35V due to the SOA (and therefore reliability) of the endstage. Furthermore, this amp really gets hot so prepare for big (!) heatsinking or just scale down the PSU a bit. It is designed for sophisticated and nice amplification. For partylevels shop elsewhere. HTH.
 
Thanks Doc, Most of my speakers play loud enough, for me, when driven by a SE EL84 amp. I'm going to try hardwiring one channel of the amp and see how my PS responds to it. If it holds up I'll try stereo. The heatsink on this amp is allready pretty large but if it gets too hot fabbing up a larger one should not be an issue. The whole back face of this chassis is one big sink but bolting on a bigger one is pretty straight foward. I just bought some of the output devices. Once they get here we'll see how it goes. I'm sure I'll have more question.
John......
 
John,

Good luck and fire away the new questions when they pop up!
I am sure you will enjoy the simple but effective design of this amp. Did you go for the MJ15003 or the 3055 transistors?
Btw. Just to warn you: in the schematic C3, the decoupling cap between endstage and loudspeaker is not drawn. It can cost you dearly when you forget to include it in your enthusiasm to connect a speaker and listen to some tunes. Ask me how I know.
 
John,

After omitting C3 I measured a short in a output track of the PCB. Had to repair it and then replaced the drivers (two BD139) and output transistors (two MJ15003) because they were dead or likely damaged. Wasn't big fun. After that everything was just fine. The difference between the 3055 and 15003 in audio quality is considerable (according to those who tried both; I immediately went for the 15003) but so is the price difference. Since you can easily exchange the devices (you have to re-check output voltage and quiescent currents of course) just first enjoy the 3055s and let us know what you think.
 
Dick, The power trany has three leads out. Two go to a bridge rectifier. The DC side of the bridge shows 36VDC. There are two 15,[email protected] caps in series that are connected to the output of the bridge. The lead connecting the two caps is also connected to the CT(I'm pretty sure) output lead of the PT. I'm not 100% sure if that gives me two 18VDC supplies or one 36VDC supply. The way the amp is wired the blown output boards currently in place see 36VDC because the CT lead never goes anywhere else after terminating at the connection between the two caps. I just assumed that they did that to make sure the voltage across both caps stayed the same, or was some sort of hum reduction, or that it was just easier to use two smaller caps instead of one big one, but I never know what's going to happen when I start assuming stuff. Tube circuits usually have resistors paralleled across seriesed PS caps when higher voltages need to be handled but these voltages seem pretty low to me. Let me know if I've got it wrong but I think the amp will still work at 18VDC.
Thanks, John.........

/This is my first SS project so if I'm getting it wrong please let me know.
 
Yes, there is considerable difference between the typical tube and solid state power supplies. Almost all solid state stuff uses a bi-polar PS, a ± pair of voltages. Most tube stuff uses a single-sided PS.

The three wires from the transformer represent 2 of AC and the third is the center tap. The center tap goes to the wire between the two large filter caps. The other two go to the diode bridge. From the diode bridge comes one lead that is + and one lead that is - in polarity.

The + lead from the bridge goes to the + side of one PS cap. The - lead from the bridge goes to the - side of the other filter cap. The connection between the two caps is the circuit or "star" ground of the amp. These two caps are not in series, they are each 1/2 of a bipolar PS and the connection between them become a zero voltage, ground point.

Although you can read 36 vdc between the + and - leads at the two filter caps, you will only find 18 vdc between the + lead and ground, and 18 vdc between the - lead and ground. Thus, this amp will run on a ±18VDC supply, not a 36 volt supply.

Study the PS circuits for solid state amps and this should become clear. As I understand tube circuits the positive is the B+ and the negative is ground. The DC voltage from the circuits is blocked by the output transformer so tube circuits can get by with a single-sided power supply. Not so with solid state stuff, unless it uses a large DC blocking capacitor between the amp's output and the speaker connection jacks. Solid state stuff uses a bipolar or ± power supply.

What a hobby! Right?

As one message implied, ±18 VDC is a good PS voltage for most Gain Clone amp circuits.
 
Dick, Here's the thing with this amp. The i/o boards definately see 36VDC each, not 18VDC. The only connection that the CT see's is the series connection between the two big caps. Is there a reason why I should not view this PS as a 36VDC supply? Just for fun I pulled the cover off my B&K ST-140. In that amp both neg leads from the PS caps are connected together so each cap feeds it's own channel. In my project amp the caps are connected in series. I just checked it again to make sure. There must be more than one way to skin this cat. I'll e-mail Fosgate-Audionic to see if they will provide a schematic but I'm not too hopefull. Are there crosstalk, or other, issues with using the PS I've described at 36VDC? I think this might be fun and some of my tube friends are already giving me a hard time about going over to the Dark Side. If this works I'll have to drive it with a tube pre or eventually build a hybred.
Thanks, John.......
 

Greg Erskine

Member
Paid Member
2002-01-05 11:56 pm
Sydney
whatsnext,

Just to make it clear, the DoZ requires a 0-36VDC supply not a 18-0-18 VDC supply.

I used a 12-0-12 toriod and just used the 2 x 12 VDC outputs in series. Remember to disconnect the centre tap from earth if it is connected. I forgot this vital step and had a few issues getting the amp to work properly, not to mention a very hot earth track and melted solder. :eek: