That is precisely the message I have been trying to preach, for years, when folk parallel two 8ohm speakers to give an effective 4ohm load. Then they ask if they can bridge a pair of amps to drive the composite 4ohm speaker.
The amps and the speakers will perform better if each 8ohm speaker is driven by it's own 8ohm capable amplifier. And this arrangement plays just as loud and when one amp or one speaker breaks you still have sound output.
You have too many diodes in the bias circuit. You only have two Vbe's (from the drivers) and then whatever you want across the output emitter resistors. Using 3 diodes is already too much. Just put a voltage meter on the emitter resistors and you will see a very, very large current in your circuit.
jd
You keep mixing up things. If you go to bridged AND KEEP THE SAME SPEAKER you get 4 x the power.
When you go to bridge but you also double the speaker impedance, you get 4 x the power from the bridging but then you get half the power because you double the speaker impedance, so the net effect is double power.
If you take your non-bridged amp and you double the speaker impedance you also get half the power. If you take your non-bridged amp and you half the speaker impedance you double the power.
power = e^2/impedance. Double e (with bridge), 4 x power. Double impedance, half power. Half impedance, double power. Double e and AT THE SAME TIME double impedance, double power.
jd
sorry very sorry mr janneman I actually was a bit confused so things happened in this manner now I understand the concept of bridging properly and why it is done ,,,,, & thanks a lot for helping me out on this .......I am still a student trying to learn sorry for being so ignorant ...
Regards
Sekhar
Don't be sorry, don't say that. You want to learn, I am happy to help you. Lots of people don't try to learn. We all had to learn, we were all born without knowledge!
jd
hey thanks lex ,
Actually i am making my amp from ground up every thing even winding my own transformers
making my own cabinet etc etc have to keep so many things in mind ...... just though that mr janneman became real angry on me how could one nt understand some thing so simple.....
neways thanks again lex .....
regards
sekhar
Actually i am making my amp from ground up every thing even winding my own transformers
making my own cabinet etc etc have to keep so many things in mind ...... just though that mr janneman became real angry on me how could one nt understand some thing so simple.....
neways thanks again lex .....
regards
sekhar
So true , janneman.
Each day, the (old) experienced people are dying and are
replaced by as much unexperience from the newly born ones..
SIR
as u r nt angry on me I have few more question I know how to bridge two amps using two opamps using one one as a inverting and another as noninverting one but how do bridge two amps by keeping one amp at a unity gain what is the advantage of such a design and is the any sonic disadvantage on normal bridging ,,,,,,
I am posting my original design any changes needed to it plz specify
An externally hosted image should be here but it was not working when we last tested it.
regards
sekhar
Bias and Bridging solutions Part 2
Okay and my apology. With this circuit start with 3 1n series 1N5817 in series and then try 4 and then 5. Please recall I do not care for gain on the output stage and there it is 7. This will bias somewhere in this range. Bypass the string of diodes with a 100uF fast cap.
Also shown is an inverting input for the amp. Trim the gain under load to achieve unity inversion. Maybe use a 953 ohm resistor and a 100 ohm trim pot in series. Measure the output of each amp to ground when it is bridged and adjust gain of the inverter so the exact same voltage is produced by both amps. I use 3kHz sine wave at 20% power 8 ohm load for this test, stable oscillator and a 4.5 digit volt meter.
Hope this covers it-
Okay and my apology. With this circuit start with 3 1n series 1N5817 in series and then try 4 and then 5. Please recall I do not care for gain on the output stage and there it is 7. This will bias somewhere in this range. Bypass the string of diodes with a 100uF fast cap.
Also shown is an inverting input for the amp. Trim the gain under load to achieve unity inversion. Maybe use a 953 ohm resistor and a 100 ohm trim pot in series. Measure the output of each amp to ground when it is bridged and adjust gain of the inverter so the exact same voltage is produced by both amps. I use 3kHz sine wave at 20% power 8 ohm load for this test, stable oscillator and a 4.5 digit volt meter.
Hope this covers it-
Attachments
you can build the two amps into a chip and sell them as a CAR AUDIO amp, where quality does not matter. All that seems to matter is how much power can be got from 13.8Vdc.
aside from quality, they are expensive to assemble, twice as many amplifiers and twice as much PSU.
There is a small saving in using lower voltage components, but this is far outweighed by the extra current capacity that needs to be built in.
You can build a 300W into 4ohm amplifier and PSU.
or you can build two 150W into 2ohm amplifiers and PSUs and bridge them for 4ohm use.
I know which is going to be less work and which is going to be cheaper.
As for the quality issue, it depends on how the bridging is done. If the amp is designed from the components up to be bridged (=balanced) then there may be improved quality if the design expertise matches the ambition.
I'm not sure I understand the question. If you want to use two power amps in bridge mode, you only need to invert the signal to one of the amps and then connect the speaker between the two 'hot' outputs of the two amps.
You can invert the signal by several methods, easiest being an inverting, unity-gain opamp stage. The sonics of the complete system will overwhelmingly be determined by the power amps so no need to worry about the opamp stage!
Does that help?
By the way, why do you want to use bridged amps actually? PhaseLockLoopy poses a good question.
jd
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here's a variant to poke at
Hey guys -- criticise the hell outta my little offering
Done in crappy old LTSpice -- unless anyone knows any other decent simulators out there I should try
This amp (see pic) is being designed specifically for load condition - driving the ribbon speakers I intend to build at 8Ω nominal.
Ribbons have very little mass in their own right - though are expected to push a huge surface area of air - which pushes up the mechanical impedance. The amplifier will be presented a load that is the mecahnical surface impedance + resistance x the input transformers turn ratio squared.
I use the above as an example of course....because ...surely in order to properly define Sekhar's lovely little amp - load is a design consideration. If how the load works on the amplifier and it is understood how load is to be driven ...doesnt the PSU become a design parameter of prime importance?
Hey guys -- criticise the hell outta my little offering
Done in crappy old LTSpice -- unless anyone knows any other decent simulators out there I should try
This amp (see pic) is being designed specifically for load condition - driving the ribbon speakers I intend to build at 8Ω nominal.
Ribbons have very little mass in their own right - though are expected to push a huge surface area of air - which pushes up the mechanical impedance. The amplifier will be presented a load that is the mecahnical surface impedance + resistance x the input transformers turn ratio squared.
I use the above as an example of course....because ...surely in order to properly define Sekhar's lovely little amp - load is a design consideration. If how the load works on the amplifier and it is understood how load is to be driven ...doesnt the PSU become a design parameter of prime importance?
Attachments
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Unity gain inverter
The first huge advantage of using a precision unity gain inverter is the distortions created by the first amp tend to be canceled by the same effect in the second amp. This is not the case if the second amp is driven by a separate inverter. Both amps being identical the distortions add together when driven by an inverter op amp. Another problem with using an inverter op amp is the delay of the op amp is added to the second channel. This has time and time again been a problem for balance transmitters in audio such that a lot of fancy schemes have been developed in the past to cause the plus and minus outputs of balanced transmitters (found on mixers and lots of pro audio gear) to remain in the same phase and amplitude relationship over the audio band. A simple inverting op amp on one side turns out to just be plain wrong.
Stabilization for an inverter amp is not the same as it is for a non-inverting amp. Someone suggested using a voltage divider on the output and then driving the inverter input with more gain. That works fine as long as gain is trimmed to 1.000.
Second big advantage is the load appears in the feedback loop of the second amp. Have a look- one side of the speaker is hooked to the amp output and the other is hooked to the inverting input. This places the load in the feedback loop which has so many obvious advantages over not having the load in the feedback I cannot count them.
For anyone who has bothered to try amplifiers with the various popular ways to bridge the channel the results are extremely clear. Using a precision trimmed unity gain inverter in both listening test and on the bench measuring distortion and damping performs the best by a wide margin. Nothing else is even close in performance.
Use two channels of a not to great an amp this way and anyone will find great improvements in performance and sound.
Power= (Voltage*voltage)/load resistance. Because this is a squared relationship doubling the voltage make 4 times the power. A perfect amp with a perfect power supply which makes 50 watts per channel in stereo will make 200 watts bridged. When the voltage is doubled then by the relationship Volts/load resistance tells us the current will also double. So the bridged amp makes 2 times the voltage and needs 2 times the current for the same load. This is exactly the same as any other two amps, one of 50 watts and one of 200 watts. Power supplies need to be scaled accordingly as example a 50 watt stereo amp (100 watts total) should have at least a 150 watt power supply. A bridged 50 watt amp making (in a perfect amplifier) 200 watts should really have at least a 300 watt power supply. Again exactly the same as any single channel 200 watt amplifier.
In a non so perfect amplifier with a good supply sized as above the 50 watt stereo will probably make about 175 watts in mono. A real soft supply and maybe 125-150. Inadequate current output ability and the amp may only make 100 watts. Add even more cheapness and the bridged amp of 50 watts will only make 75 watts in bridge. All this into the same 8 ohm load.
So to name the advantages of bridging as above:
1. Less voltage across all devices so large SOA is achieved.
2. Load appears in feedback loop greatly reducing distortion.
3. Amplifier distortions are tend to be canceled and not added as in other schemes.
4. Damping of load is much higher than with any other scheme.
5. Lower voltage components may be used allowing much faster devices to be chosen.
6. Sound quality is grossly better because of the distortion canceling.
7. Reliability is much greater because of the SOA (Safe area of operation) of the output transistor is much larger.
Bridging disadvantage:
1. Likely more parts in the amp than the equivalent single amp with the same power rating as the bridged amp.
2. The requirement to trim one amp to exactly unity gain inverter.
3. No ground referenced signal at the speaker output. (this does not usually matter but is important in some applications)
In the end I run all amps in bridge mode. I have solved the need to trim by using matched precision 0.1% resistors and very high gain amplifier. My 55 watt amp has 225dB of gain before feedback. With the bridge gain before feedback is 450dB. Yes this is correct and has been measured directly.
So bridge if you like or not. For me and everyone I know closely bridging is used as often as is practical. The few extra parts and small amount of trouble has proved over and over to be such a huge advantage in reliability and sound clarity that at this point there is not real question. I even make 15 watt amps and bridge them to get 50 watts rather than the single 50 watt amp because they sound and work so much better.
The first huge advantage of using a precision unity gain inverter is the distortions created by the first amp tend to be canceled by the same effect in the second amp. This is not the case if the second amp is driven by a separate inverter. Both amps being identical the distortions add together when driven by an inverter op amp. Another problem with using an inverter op amp is the delay of the op amp is added to the second channel. This has time and time again been a problem for balance transmitters in audio such that a lot of fancy schemes have been developed in the past to cause the plus and minus outputs of balanced transmitters (found on mixers and lots of pro audio gear) to remain in the same phase and amplitude relationship over the audio band. A simple inverting op amp on one side turns out to just be plain wrong.
Stabilization for an inverter amp is not the same as it is for a non-inverting amp. Someone suggested using a voltage divider on the output and then driving the inverter input with more gain. That works fine as long as gain is trimmed to 1.000.
Second big advantage is the load appears in the feedback loop of the second amp. Have a look- one side of the speaker is hooked to the amp output and the other is hooked to the inverting input. This places the load in the feedback loop which has so many obvious advantages over not having the load in the feedback I cannot count them.
For anyone who has bothered to try amplifiers with the various popular ways to bridge the channel the results are extremely clear. Using a precision trimmed unity gain inverter in both listening test and on the bench measuring distortion and damping performs the best by a wide margin. Nothing else is even close in performance.
Use two channels of a not to great an amp this way and anyone will find great improvements in performance and sound.
Power= (Voltage*voltage)/load resistance. Because this is a squared relationship doubling the voltage make 4 times the power. A perfect amp with a perfect power supply which makes 50 watts per channel in stereo will make 200 watts bridged. When the voltage is doubled then by the relationship Volts/load resistance tells us the current will also double. So the bridged amp makes 2 times the voltage and needs 2 times the current for the same load. This is exactly the same as any other two amps, one of 50 watts and one of 200 watts. Power supplies need to be scaled accordingly as example a 50 watt stereo amp (100 watts total) should have at least a 150 watt power supply. A bridged 50 watt amp making (in a perfect amplifier) 200 watts should really have at least a 300 watt power supply. Again exactly the same as any single channel 200 watt amplifier.
In a non so perfect amplifier with a good supply sized as above the 50 watt stereo will probably make about 175 watts in mono. A real soft supply and maybe 125-150. Inadequate current output ability and the amp may only make 100 watts. Add even more cheapness and the bridged amp of 50 watts will only make 75 watts in bridge. All this into the same 8 ohm load.
So to name the advantages of bridging as above:
1. Less voltage across all devices so large SOA is achieved.
2. Load appears in feedback loop greatly reducing distortion.
3. Amplifier distortions are tend to be canceled and not added as in other schemes.
4. Damping of load is much higher than with any other scheme.
5. Lower voltage components may be used allowing much faster devices to be chosen.
6. Sound quality is grossly better because of the distortion canceling.
7. Reliability is much greater because of the SOA (Safe area of operation) of the output transistor is much larger.
Bridging disadvantage:
1. Likely more parts in the amp than the equivalent single amp with the same power rating as the bridged amp.
2. The requirement to trim one amp to exactly unity gain inverter.
3. No ground referenced signal at the speaker output. (this does not usually matter but is important in some applications)
In the end I run all amps in bridge mode. I have solved the need to trim by using matched precision 0.1% resistors and very high gain amplifier. My 55 watt amp has 225dB of gain before feedback. With the bridge gain before feedback is 450dB. Yes this is correct and has been measured directly.
So bridge if you like or not. For me and everyone I know closely bridging is used as often as is practical. The few extra parts and small amount of trouble has proved over and over to be such a huge advantage in reliability and sound clarity that at this point there is not real question. I even make 15 watt amps and bridge them to get 50 watts rather than the single 50 watt amp because they sound and work so much better.
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DC offset DC servo
Think you will need a DC servo to control DC offset to minimum values under all conditions. Ribbons do not like even a little DC as in more than about a millivolt.
Hey guys -- criticise the hell outta my little offering
Think you will need a DC servo to control DC offset to minimum values under all conditions. Ribbons do not like even a little DC as in more than about a millivolt.
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