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Old 29th January 2018, 02:01 AM   #5061
Keremito is offline Keremito  United States
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Default High power supply ripple. Hints?

I built a F5T V2 with dual mono PSU's using 2 of the V3 boards from the store. Currently running at +/- 40 V DC rails with 2 300 VA transformers. Following Nelson's diagram, I implemented a ground isolation system with a diode bridge, thermistor and capacitor.
The amplifier sounds great with the exception that it presents a low level of noise that can be heard at the mid range and tweeter drivers.
There is no humming at the woofer.
Each dual rail power supply has 120,000 uF of filtering. 8 15,000 uF capacitors 50 V DC. Using and oscilloscope I can see a ripple of around 32 mV rms, 120 Hz with the amplifier boards connected. It drops to almost zero with no load.
What do you recommend to do to improve the amplifier?
Thanks in advance for replies.
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Old 29th January 2018, 08:30 AM   #5062
AndrewT is offline AndrewT  Scotland
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You say there is no hum from the bass speakers and that there is noise from the Mid and Treble speakers.

Seems noise is your problem, not hum.
The measurements you give for ripple are related to minimising hum.

Have you measured the Hum+Noise at the amplifier output?
What are the sensitivities of the three speaker drivers?

Do this with the inputs shorted with zero ohm dummy plugs.
Repeat with long interconnects, again shorted at the far end with zero ohms dummy sockets.
Repeat with long interconnects to a source with the vol pot turned right down.
Post all the results.
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Last edited by AndrewT; 29th January 2018 at 08:37 AM.
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Old 30th January 2018, 07:48 AM   #5063
Keremito is offline Keremito  United States
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Thanks Andrew. I suspect a grounding issue. Last night I managed to blow the output FETs on one channel. Amp is back in service. While on the bench:
DC offset with open RCA input and no speakers connected: less than 50 mV. I connected a shorted RCA male to the input, the DC offset jumped to a bit more than 0.5 Volts.
If I plugged an interconnect to the input the DC offset jumped to a bit under 0.5 Volts even with the other end in the air, about the same if connected to a source. Connecting speakers did not change these values much.
I adjusted the DC offset with the input shorted, below 30 mV now, does not change much with the speakers connected. This remains true as long as there is an inout connected, when an input is removed, the DC offset goes up to around 0.5 V. What is happening?
I don't know how to measure Hum+noise, what tool(s) do I need for this?
The speakers are NHT 2.9's sensitivity is specified at 87 db.
Reports on test suggested:
With open inputs the amplifier is quiet.
With shorted dummy, noise is present but not low.
With short interconnects more noise.
With long unshielded noise is stronger with open end, stronger with cable in vertical position, less in horizontal position. (RF interference?)
With long cable if a short de end to plugged to the amplifier, noise drops to low level.
With cable connected to a source is quieter with short shielded cables than unshielded and long cables.
If source is off but connected to amp the level of noise is greater.
Next thing to try tomorrow is the install two ground rods to improve the grounding system of the electrical system of the house. Isolated grounding? May be, I will keep chasing. I think is a grounding issue... we will see.
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Old 31st January 2018, 07:51 AM   #5064
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Keremito View Post
Thanks Andrew. I suspect a grounding issue. Last night I managed to blow the output FETs on one channel. Amp is back in service. While on the bench:
DC offset with open RCA input and no speakers connected: less than 50 mV. I connected a shorted RCA male to the input, the DC offset jumped to a bit more than 0.5 Volts.
If I plugged an interconnect to the input the DC offset jumped to a bit under 0.5 Volts even with the other end in the air, about the same if connected to a source. Connecting speakers did not change these values much.
I adjusted the DC offset with the input shorted, below 30 mV now, does not change much with the speakers connected. This remains true as long as there is an inout connected, when an input is removed, the DC offset goes up to around 0.5 V. What is happening?
I see two possibilities.
Either:
a.) the amp is oscillating when the open input has an aerial picking up interference attached,
or
b.) You have a DC coupled input and the source "resistance" seen by the input changes when you change the source connection.
Quote:
I don't know how to measure Hum+noise, what tool(s) do I need for this?
You need an AC voltmeter. I use a variety of cheap digital voltmeters that have a 199.9mVac scale and even though they are not accurate they give a very good approximation to the H+N at the amplifier output.
A 1.999Vac scale is not sensitive enough to get reasonable estimates of output H+N.
Quote:
The speakers are NHT 2.9's sensitivity is specified at 87 db.
Reports on test suggested:
With open inputs the amplifier is quiet.
With shorted dummy, noise is present but not low.
These two results are the opposite to what we usually find for a normal power amplifier. The quietest operation is usually with a zero ohms input plug fitted to each input.
Quote:
With short interconnects more noise.
An interconnect with an open remote end acts as an aerial picking up interference. This is generally noisier than any other connection.
Quote:
With long unshielded noise is stronger with open end, stronger with cable in vertical position, less in horizontal position. (RF interference?)
Yes, the shield is the return route for current entering from a source. Where no source is connected the interconnect core acts as an interference pick up aerial. The screen does not provide effective shielding for an open ended core.
Quote:
With long cable if a short de end to plugged to the amplifier, noise drops to low level.
This is normal. The short at the remote end is a test I recommend when testing a stereo amplifier. Connecting two long interconnects is the next stage. then shorting the remote ends together is the third stage. shorting the remote end screens together is a test to find your amplifiers tolerance to being connected to a stereo source that has a common "ground" for the signal returns.
Quote:
With cable connected to a source is quieter with short shielded cables than unshielded and long cables.
This is a bit unusual. With an interconnect cable terminated at both ends, it normally makes little to no difference to the noise from the power amplifier. Maybe one of your interconnect cables is faulty?
Quote:
If source is off but connected to amp the level of noise is greater.
This is quite common. An OFF source can present any of a variety of source impedance to the input cable and the input stage. If the OFF source impedance is higher than Zero ohms, then some noise will be added to the input and may become audible, certainly measurable.
Quote:
Next thing to try tomorrow is the install two ground rods to improve the grounding system of the electrical system of the house. Isolated grounding? May be, I will keep chasing. I think is a grounding issue... we will see.
NO !!!!!!!!
Do not alter a correctly installed mains distribution system.
If you suspect a fault in your mains system get a qualified electrician to come and check your installation.

The reason for NOT adding Earthing Rods is due to the voltages and currents in your locality during an electrical storm. With the correctly installed earthing system that the mains electricity distribution company has provided, the whole house and you and all your wiring is at the same voltage during a lightning strike nearby.
You must NOT interfere with this. Your life depends on that equality of voltage during the nearby lightning strike.
If anyone installs extra Earthing Rods then there exists the strong probablility that two different voltages can appear at the two Earth rod installations and you end up with differing voltages between the two rods during a lightning strike.
If you were unfortunate enough to be very close to a high potential difference (say an output socket relative to the floor of your house) you could get a lightning arc from the socket to you, through you to your floor. You have added an unauthorised and untested earthing rod that bring the lightning INSIDE your house !
Similarly electrical equipment connected between the two differing Earth Rod installation could end up with a high voltage between them and get damaged/destroyed.
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Last edited by AndrewT; 31st January 2018 at 08:06 AM.
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Old 3rd February 2018, 11:42 PM   #5065
Keremito is offline Keremito  United States
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Andrew, thank you for your reply. I have no gotten back to it due to work. I will dig in a bit more with different internal wire layouts and make sure that I have a good grounding system to sink noise before it reaches the amp's input. I am using long (1.5 m) Kimber unshielded interconnects, these may have to go as well if found to be the problem.
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Old 9th February 2018, 10:47 AM   #5066
coolnose is offline coolnose  Europe
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Quote:
Originally Posted by Zen Mod View Post
bias is hardly an issue , if you got it in proper temp equilibrium

what you need to triple check is oscillation - maybe is best to mount (increase) those lag caps in feedback network

Papa is Old Devil with zillion miles under the bu.....m , having entire circuit on one pcb , with controlled patterns thus capacitance of traces etc. , so he can do it without lag caps

put some 3n9-4n7 caps across R9 and R7 , be sure that preamp is flawless regarding transients and oscillation , simply because F5 is pretty much Lucky Luke fast

and - install decent speaker protection modules

something with μPC1237 will do ...... you can even make it as separate unit , with internal PSU

I would shoot with shotgun any amplifier responsible for burning my precious RCAs
A good idea to use those styroflex I have laying around ?
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Old 9th February 2018, 12:21 PM   #5067
coolnose is offline coolnose  Europe
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Quote:
Originally Posted by Zen Mod View Post
33Vdc rails

1mA through divider is enough

use 12K for R27/R28 and 18K for R25/R26 (ref. to F5Turbo V3 , page 13/17 of article)



you'll have +/-13V2 at cascode bases , and around +/-12V6 at JFet drains
Zen Mod,

sorry for digging this old post out, but do I understand it right that you aim for the lowest current through R27/R28 ? Noise ?

Thanks,

Max
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Old 9th February 2018, 03:05 PM   #5068
Zen Mod is offline Zen Mod  Serbia
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F5 Turbo Builders Thread
Quote:
Originally Posted by coolnose View Post
A good idea to use those styroflex I have laying around ?

if they're good , why not

if Papa can use them , you can too

though , I don't like tubular stryroflex


Quote:
Originally Posted by coolnose View Post
Zen Mod,

sorry for digging this old post out, but do I understand it right that you aim for the lowest current through R27/R28 ? Noise ?

Thanks,

Max
no need for more ........ impedances are higher , thus filtering less critical (cap from bases to gnd)
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Old 9th February 2018, 05:35 PM   #5069
coolnose is offline coolnose  Europe
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Thanks a lot once more demi-dieu !

Max
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Old 19th February 2018, 04:10 PM   #5070
KyleMD is offline KyleMD  United States
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Default Low Bias - R5/R6 resistor change suggestions

I am building the Turbo V2 with quad matching Toshiba JFETS from punkydawgs on eBay (ostensibly 6.5mA-7.9mA Idss, although I haven't been able to measure) and standard 32V PSU rails. I am using the Deluxe 5U chassis.

I am having trouble with low max bias measurements. On one channel, I was only able to measure a max 170mV of bias across the source resistors with 0 DC offset, while on the other channel I was able to max out at ~190mv with 0 DC offset. These measurements were taken with inputs shorted and with a standard power cable attached (no light bulb). The amplifier works with these settings and gets warm to the touch, but the bias is not optimal. (There is also a very slight 60hz hum that I believe I can eliminate with some layout optimization.)

I understand that I can boost the max bias by increasing the values of R5/R6 from the standard 1Kohm. Does anybody have any suggestion for a starting point for the R5/R6 resistor change? My target bias is 3.5mv, so essentially double what I am able to measure right now. I suppose I would be willing to settle for anything over 3mV.

Another thing I noticed that may be significant is that I did not observe the "2 steps forward, 1 step back" phenomena described in the build guide when I adjusted bias. One knob would increase bias and take DC offset below zero, as described in the build guide. When I took zeroed DC offset with the other knob, however, bias would continue to increase.
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