hi Chris,
Do you think it would be too risky to run two 555's bridged for my set up (6 ohm).
I don't think I would try to run it wide open (try to limit Vin max). Just looking for maybe 2-3 dB head room to keep away from clipping. say at Po=500W RMS max. Is there anything I could do to safely run bridged (4ohm)? Upgrade drivers/output devices Re's etc. Don't really want to lose my speakers under any conditions.
Do you think it would be too risky to run two 555's bridged for my set up (6 ohm).
I don't think I would try to run it wide open (try to limit Vin max). Just looking for maybe 2-3 dB head room to keep away from clipping. say at Po=500W RMS max. Is there anything I could do to safely run bridged (4ohm)? Upgrade drivers/output devices Re's etc. Don't really want to lose my speakers under any conditions.
I've run a bridged 555 into a speaker that measured 4 ohms DCR. It will do 900W into 4 ohms until it gets hot.
900W is about 25W per output device, about the maximum for the stock 150W parts.
The driver transistors will go first.
When driving 4 ohms in mono and you blow the fuses, the driver transistors will almost always blow.
MJE150XX will take care of that (this is the part originally specified by Nelson Pass).
I have replaced the stock 700VA toroid with a 1.8KVA E&I type before, it makes a world of difference. I have also used a pair of 1KVA toroids. Just stack them up with the provided washers and plates, with a longer bolt.
Talema makes nice toroids, I own a pair of 625VA models made in their USA plant. I'm not sure there is room in a 545 for a bigger transformer, but there is plenty of room in the 7" tall 555 case.
The dual mono supplies are non-optimum for bridged use, you're better off with a mono supply.
If you pound it hard with a beefed-up supply, be sure an hook up a pair of fans too.
It doesn't hurt to put in 250W ON Semiconductor outputs either.
900W is about 25W per output device, about the maximum for the stock 150W parts.
The driver transistors will go first.
When driving 4 ohms in mono and you blow the fuses, the driver transistors will almost always blow.
MJE150XX will take care of that (this is the part originally specified by Nelson Pass).
I have replaced the stock 700VA toroid with a 1.8KVA E&I type before, it makes a world of difference. I have also used a pair of 1KVA toroids. Just stack them up with the provided washers and plates, with a longer bolt.
Talema makes nice toroids, I own a pair of 625VA models made in their USA plant. I'm not sure there is room in a 545 for a bigger transformer, but there is plenty of room in the 7" tall 555 case.
The dual mono supplies are non-optimum for bridged use, you're better off with a mono supply.
If you pound it hard with a beefed-up supply, be sure an hook up a pair of fans too.
It doesn't hurt to put in 250W ON Semiconductor outputs either.
Hi Infinia,
If you rebuild the amps as djk has suggested (and I agree 100% with him on this) the amplifier would be safer to run 4 ohm bridged. If you do not do this the drivers will fail and take out the outputs along with them. The speakers may be damaged in the process.
Understand that by upgrading the power supply you will have about 3~3.5 X the 4 ohm power of one channel. You don't want to see the distortion lights come on. You will need to increase the ventilation and fans may be part of this. Along with fans come dust build up. Do not fail to clean everything out from time to time.
You want to use MJ21195 and MJ21196 for the higher SOA ratings. Match them if you can. I wonder if you can get MJL3281 and MJL1302 to fit in as driver transistors. They would stand up better.
-Chris
If you rebuild the amps as djk has suggested (and I agree 100% with him on this) the amplifier would be safer to run 4 ohm bridged. If you do not do this the drivers will fail and take out the outputs along with them. The speakers may be damaged in the process.
Understand that by upgrading the power supply you will have about 3~3.5 X the 4 ohm power of one channel. You don't want to see the distortion lights come on. You will need to increase the ventilation and fans may be part of this. Along with fans come dust build up. Do not fail to clean everything out from time to time.
You want to use MJ21195 and MJ21196 for the higher SOA ratings. Match them if you can. I wonder if you can get MJL3281 and MJL1302 to fit in as driver transistors. They would stand up better.
-Chris
Thanks Chris and djk,
So to upgrade for 555 mkI bridged mode into 4 ohm
The changes are proposed as follows
Drivers = mje15030/31 drivers (mandatory)
Double 800VA toroids 55+55 Vac (or 1.5KVA single EI)
Re=0R33 5W (mandatory)
Outputs=Mj21195/96 (optional)
Inrush current limit =parallel 2 CL-30's shorted with 18A relay
Dual fans on outside HS if used in HD environment
stock 15KuF Caps OK
stock Diode bridge OK
So to upgrade for 555 mkI bridged mode into 4 ohm
The changes are proposed as follows
Drivers = mje15030/31 drivers (mandatory)
Double 800VA toroids 55+55 Vac (or 1.5KVA single EI)
Re=0R33 5W (mandatory)
Outputs=Mj21195/96 (optional)
Inrush current limit =parallel 2 CL-30's shorted with 18A relay
Dual fans on outside HS if used in HD environment
stock 15KuF Caps OK
stock Diode bridge OK
You guys... you get so involved with fixin what simply isn't worth owning let alone upgrading. Sure they can be brought up to a semblance of a current performer.. but why??
In the Used market there are Lotsa better sounding more capable units than old discredited Adcoms.. this Ain't 1988 any more.
In the Used market there are Lotsa better sounding more capable units than old discredited Adcoms.. this Ain't 1988 any more.
Thanks a lot!
I popped those ONSEMIS in when I recv'd the amp with a blown side. I had no idea if they were suitable replacements but they seem to be fine, if not better than the tosh's. When I saw what peeps were getting for the originals I almost puked
. This amp may<or>will be used for sub duty..no more.
I popped those ONSEMIS in when I recv'd the amp with a blown side. I had no idea if they were suitable replacements but they seem to be fine, if not better than the tosh's. When I saw what peeps were getting for the originals I almost puked
. This amp may<or>will be used for sub duty..no more.
Is it best to pass (pun intended) on trying to get a pair of 555's to do bridge. Not sure what the failure mechanisms are but seems like alot of amps can smoke in bridge mode as well. It doesn't seem intuitive that adding double power transformers would make this any safer. any thoughts appreciated.
Hi Infinia,
I have never been a fan of running amps in bridged mode. There are bigger amps available and you get way more mileage running one channel per woofer. That means adding a woofer in some cases.
I agree with Nelson's observation that the 555 is not a great amp in bridge mode. It wasn't designed for that duty. Besides, Adcom made the 565 as a mono amp.
All in all you can do a whole lot worse than these amps. Just use them as they were intended to be used.
-Chris
I have never been a fan of running amps in bridged mode. There are bigger amps available and you get way more mileage running one channel per woofer. That means adding a woofer in some cases.
I agree with Nelson's observation that the 555 is not a great amp in bridge mode. It wasn't designed for that duty. Besides, Adcom made the 565 as a mono amp.
All in all you can do a whole lot worse than these amps. Just use them as they were intended to be used.
-Chris
Two things make it less that ideal for mono use, the dual mono supply, and the method of bridging.
The method of the 555 used for bridging is the best from a stability standpoint, but the worst from a sound standpoint. It would be fine for subwoofer use(I would strap the supply to mono too).
The method of the 555 used for bridging is the best from a stability standpoint, but the worst from a sound standpoint. It would be fine for subwoofer use(I would strap the supply to mono too).
Thanks for the replies Guys,
So no bridging the 555 even though the capability was designed into it.
I need to add the slow start circuit (inrush I limit) so at the same time maybe I could manage to change the drivers. If anybody could remember offhand. How involved would it be to change the driver transistors? I haven't peaked inside since i first bought it new. If I have to unsolder all the TO-3 leads I'll probably pass on that as well.
I have an idea for the slow start circuit. I have a SPDT 125Vdc 18A relay. The relay windings will be across both rails at the supply with a small series resistor and buck diode. The NO contacts shorting a small power thermistor in the primary circuit.
Nothing I have seen is simpler than that.
So no bridging the 555 even though the capability was designed into it.
I need to add the slow start circuit (inrush I limit) so at the same time maybe I could manage to change the drivers. If anybody could remember offhand. How involved would it be to change the driver transistors? I haven't peaked inside since i first bought it new. If I have to unsolder all the TO-3 leads I'll probably pass on that as well.
I have an idea for the slow start circuit. I have a SPDT 125Vdc 18A relay. The relay windings will be across both rails at the supply with a small series resistor and buck diode. The NO contacts shorting a small power thermistor in the primary circuit.
Nothing I have seen is simpler than that.
Hi Infinia,
About bridging ... "just because you can, doesn't mean you should". One of the most misunderstood and misused features.
The driver and predriver transistors share a mounting bolt. The leads are soldered to posts standing off the PCB. So, no problem replacing them.
Use a fixed resistor for the surge resistor. There is no point in a thermistor at all. Otherwise, it's that simple.
-Chris
About bridging ... "just because you can, doesn't mean you should". One of the most misunderstood and misused features.
The driver and predriver transistors share a mounting bolt. The leads are soldered to posts standing off the PCB. So, no problem replacing them.
Use a fixed resistor for the surge resistor. There is no point in a thermistor at all. Otherwise, it's that simple.
-Chris
Hi Infinia,
If you have them, go ahead and use them.
1R0 is too low. Common values are 2R2 (big amps) to 10R. What I like about a fixed resistor is that during a fault condition, if the relay drops out the resistor will burn out. Added safety. Your part will not probably, so the power is not interrupted.
-Chris
If you have them, go ahead and use them.
1R0 is too low. Common values are 2R2 (big amps) to 10R. What I like about a fixed resistor is that during a fault condition, if the relay drops out the resistor will burn out. Added safety. Your part will not probably, so the power is not interrupted.
-Chris
I've repaired a monster reciever with 0R5 5W and the part goes faster than any correct fuse would. So I doubled that value for a swag. If you make R too big it can fail under normal conditions. *ell any resistance is better than stock yes.
BTW I love the smell of a cooking thermistor in the morning. It smells like relay failure.
&thanks for your help
BTW I love the smell of a cooking thermistor in the morning. It smells like relay failure.
&thanks for your help
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