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Best practice before switching on new equipment

D

dspatterson

pfm Member
Hi, after reading the Avi power amp post, and just receiving my own power amp today from a forum member, I thought it might be usefull to pull together tips and ideas that just may save someone stress now or in the future.

Firstly we all have different levels of knowledge and expertise of electrical equipment.
I can only comment from a position of, non technically trained ( prob with all due respect guys, a fair proportion of us)
More knowledgeable folk please feel free to add any usefull tips.

First thing I do is look at the the condition of the container it travelled in. If there is any obvious damage intrusions openings photograph there and then before opening. Use a time dated photo.

Once component is out of box again 360 degree check, in natural light, it's amazing what shows up under different lighting.
(a purple anodised fascia on a Naim amp looked great in artificial light)
Obviously looking for physical damage.
If you do not feel confidant opening or removing casework due to invalidating guarantee or causing damage I would rotate device in all planes, this can usually show up loose screws or swarf within case. ( again I've had both). Seek advise if required.
One thing I always do, especially living up here in Scotland is allow the new device to come to room temp.
The amp I received today measured minus 2degs on my ir thermometer. I may be being ott here but I just don't want to power rush my stuff at that temp.
Also cold items attact condensation when brought into warm moist environment.
Like many we dry allot of towels clothes on radiators at this time of year and that can put a hell of allot of moisture into your home.
So I let the device time to come to room temp. A challenge when we are all keen to hear our next best thing.

I am not technically trained and would not know what to look for on a meter on the outputs, but I do power up the item with nothing connected and allow it to operate for a short time, again I've had smoke after about a minute once, causing mad rush to the on off (at the wall socket)
Once I'm happy i will power down, connect up and switch on, but I'm still really cautious when connecting speakers.
 
The only really important thing is to check the output with a multimeter to make sure there is no horrible offset present. This can only happen with solid state amps and then only direct coupled ones.
Don't **** it up by setting the meter to read current!!!! This will usually blow up the amp! Fortunately with most meters you have to physically move the lead to the current socket... but not all...
Set meter to DC Volts. If non auto-ranging or analogue meter then 20V ish scale is appropriate. If amp is OK then there will be the thin end of bugger all coming from it, usually in the 1mV to 50mV range (0.001 - 0.050V)

As I said valve amps are not relevant here. They are safe.... for speakers anyway! Capacitor coupled solid state amps are also speaker safe in as much as cannot pump DC into speakers and fry the woofers. It will ALWAYS be woofers that are damaged by this BTW. Capacitor coupled solid state amps are virtually unheard of these days though and were gradually phased out by around '75 ish.... About the only "modern" one I can think of was the Audio Innovations Alto.
One possible confusion to be aware of is that whilst capacitor coupled solid state amps can't put DC into your speakers they will often (not always, but most will) read as having a huge DC offset on a meter if checked without speakers! That big "woomph" that you usually get with capacitor coupled solid state amps at switch on is the output capacitor charging up to half the DC supply voltage through your speaker (don't worry. sounds worse than it is;)) and without a speaker present "to complete the circuit" you will usually measure a large DC offset which gradually falls to near zero over a few seconds to a few minutes. Confusing for a noob I guess...

All solid state amps are fine without speakers connected and virtually all valve amps used in hi fi will be fine as well but if for some bizarre reason you do switch on a valve amp without speakers then have vol on zero just to be sure.

A short circuit across a valve amps output will not bother it but a short across a solid state amps output WILL destroy it instantly unless it has suitable protection circuitry.

Some of the more common capacitor coupled solid state amps include:

Quad 303.
Sugden A21 (original 60's early 70's type)
Sugden A48
Armstrong 400, 500 and 600 series
Audio innovations Alto
Rogers Ravensbrook and Ravensbourne
Nytech CTA252
Creek 4040 S1
Leak Stereo 30, 30 plus, 70 plus, delta 30, delta 70, delta 75

If it's made before 1971 it will probably be capacitor coupled.... but there are exceptions!
 
Last edited:
Dartzeel solid state power amps go bang if powered up without speakers connected.
Probably not an issue for most forum meters though :)
 
No, iirc it was noted in Sterophile's / J Atkinsons measurement of the first Dartzeel effort - internal fuse went.
But then Dartzeel are far distance up the 'wilfull/wtf?!' axis of SS amp design; things like no emitter R in the (BJT) output stage for more purity, nonsense like that.
 
martin clark said:
No, iirc it was noted in Sterophile's / J Atkinsons measurement of the first Dartzeel effort - internal fuse went.
But then Dartzeel are far distance up the 'wilfull/wtf?!' axis of SS amp design; things like no emitter R in the (BJT) output stage for more purity, nonsense like that.
Click to expand...

So unlikely I'd have to see it myself to believe it... the level of incompetence it would take in design boggles the mind.... like it being "tetchy" anyway and then not fitting a Zobel network.. especially if it's common emitter at the output! Open loop gain through the roof then as soon as you remove the load if no Zobel...Bye bye gain margin... hello Mr Barkhausen... Even Naim fit a Zobel FFS!
 
Arkless Electronics said:
The only really important thing is to check the output with a multimeter to make sure there is no horrible offset present. This can only happen with solid state amps and then only direct coupled ones.
Don't **** it up by setting the meter to read current!!!! This will usually blow up the amp! Fortunately with most meters you have to physically move the lead to the current socket... but not all...
Set meter to DC Volts. If non auto-ranging or analogue meter then 20V ish scale is appropriate. If amp is OK then there will be the thin end of bugger all coming from it, usually in the 1mV to 50mV range (0.001 - 0.050V)

As I said valve amps are not relevant here. They are safe.... for speakers anyway! Capacitor coupled solid state amps are also speaker safe in as much as cannot pump DC into speakers and fry the woofers. It will ALWAYS be woofers that are damaged by this BTW. Capacitor coupled solid state amps are virtually unheard of these days though and were gradually phased out by around '75 ish.... About the only "modern" one I can think of was the Audio Innovations Alto.
One possible confusion to be aware of is that whilst capacitor coupled solid state amps can't put DC into your speakers they will often (not always, but most will) read as having a huge DC offset on a meter if checked without speakers! That big "woomph" that you usually get with capacitor coupled solid state amps at switch on is the output capacitor charging up to half the DC supply voltage through your speaker (don't worry. sounds worse than it is;)) and without a speaker present "to complete the circuit" you will usually measure a large DC offset which gradually falls to near zero over a few seconds to a few minutes. Confusing for a noob I guess...

All solid state amps are fine without speakers connected and virtually all valve amps used in hi fi will be fine as well but if for some bizarre reason you do switch on a valve amp without speakers then have vol on zero just to be sure.

A short circuit across a valve amps output will not bother it but a short across a solid state amps output WILL destroy it instantly unless it has suitable protection circuitry.

Some of the more common capacitor coupled solid state amps include:

Quad 303.
Sugden A21 (original 60's early 70's type)
Sugden A48
Armstrong 400, 500 and 600 series
Audio innovations Alto
Rogers Ravensbrook and Ravensbourne

If it's made before 1971 it will probably be capacitor coupled.... but there are exceptions!
Click to expand...

If I understand you correctly, capacitor coupled is broadly ‘safer’ but now largely a thing of the past? Which then begs the question why - did amp design get better in other ways so it was no longer felt to be necessary or was it more down to a marginal extra parts cost?
 
I think you will find that capacitor coupled amps died because they had a capacitor directly in line with the loudspeakers! If you remember back to the the 1980s and 1990s capacitors were a dirty word! If would have been very difficult to sell something when the hifi press was constantly telling everyone that capacitors cause huge issues.
On a more technical basis one of the disadvantages of capacitor coupling is that the power supply capacitors have to have a higher voltage rating than the normal type of amplifier with + and - rails. The capacitor coupled amps only have a +ve rail which means that 80V rated capacitors would have to be used if you wanted to make an amplifier similar to a +-35V twin rail amp. 80V capacitors are considerably more expensive than 35V ones. From a marketing perspective it is also better to quote 100000µF power supply capacity rather than 30000µF. The 30000µF 80V capacitors store about the same energy as 100000µF 35V, but that doesn't sound as good on the sales leaflet!
If you can remember back to the old Nytech amps, capacitor coupled, they had a reputation for a "woolly" bass. This was resolved on the following Ion Systems (Nytech follow on company) amps which had much larger capacitance output capacitors but I think the "damping factor" was still not as good as conventional +- amps and thus had a tendency to still sound "woolly" on poorly designed ported loudspeakers (the loudspeakers fault!).
However if you were to use a capacitor coupled amp in an active system with closed box speakers (the old Nytech/Arc systems) then you get an increase in lower bass extension compared to a +- railed amp.

I personally use capacitor coupled Ion System amps and find them very musically satisfying.

But getting back to the original post you must remember not to disconnect the speaker cables from the amplifer until one or two minutes after turning off as it takes a long time for all the capacitors to discharge. If you do it too quickly you could potentially cause some damage.
 
cpg said:
I think you will find that capacitor coupled amps died because they had a capacitor directly in line with the loudspeakers! If you remember back to the the 1980s and 1990s capacitors were a dirty word! If would have been very difficult to sell something when the hifi press was constantly telling everyone that capacitors cause huge issues.
On a more technical basis one of the disadvantages of capacitor coupling is that the power supply capacitors have to have a higher voltage rating than the normal type of amplifier with + and - rails. The capacitor coupled amps only have a +ve rail which means that 80V rated capacitors would have to be used if you wanted to make an amplifier similar to a +-35V twin rail amp. 80V capacitors are considerably more expensive than 35V ones. From a marketing perspective it is also better to quote 100000µF power supply capacity rather than 30000µF. The 30000µF 80V capacitors store about the same energy as 100000µF 35V, but that doesn't sound as good on the sales leaflet!
If you can remember back to the old Nytech amps, capacitor coupled, they had a reputation for a "woolly" bass. This was resolved on the following Ion Systems (Nytech follow on company) amps which had much larger capacitance output capacitors but I think the "damping factor" was still not as good as conventional +- amps and thus had a tendency to still sound "woolly" on poorly designed ported loudspeakers (the loudspeakers fault!).
However if you were to use a capacitor coupled amp in an active system with closed box speakers (the old Nytech/Arc systems) then you get an increase in lower bass extension compared to a +- railed amp.

I personally use capacitor coupled Ion System amps and find them very musically satisfying.

But getting back to the original post you must remember not to disconnect the speaker cables from the amplifer until one or two minutes after turning off as it takes a long time for all the capacitors to discharge. If you do it too quickly you could potentially cause some damage.
Click to expand...
OK thanks, makes sense. Just to extend that a little further: my power amps (Tron Convergence) each have two separate inputs, one capacitor coupled the other direct. Again the coupled is considered 'safer' but the direct 'better' - as yet I have not tried the direct but in the spirit of the original post: would there be additional sensible precautions when using a direct coupled power amp input? I guess this partly depends on the pre amp output (and yes I did ask Graham Tricker when I first got the amps but having had them a fair while feel I might now be in a better position to understand the answer.)
 
Be VERY careful when using the "direct" input of your power amplifier. The capacitor in the input line is there to elminate any DC signal from the preamp being amplified in the power amp. If your were to feed your power amp a DC signal this would be amplified and cause your speakers cones to travel way out of specification and quite probably burn out the coils in the drive units.
If you read some of the Naim DIY threads on PFM then you will see that some people have tried removing the input capacitors of their power amps but I am sure they have checked with a multimeter beforehand that NO DC component is coming from the pre-amp.
 
cpg said:
Be VERY careful when using the "direct" input of your power amplifier. The capacitor in the input line is there to elminate any DC signal from the preamp being amplified in the power amp. If your were to feed your power amp a DC signal this would be amplified and cause your speakers cones to travel way out of specification and quite probably burn out the coils in the drive units.
If you read some of the Naim DIY threads on PFM then you will see that some people have tried removing the input capacitors of their power amps but I am sure they have checked with a multimeter beforehand that NO DC component is coming from the pre-amp.
Click to expand...
Then again, does the same apply to a pre-amp, that it may or may not have output capacitors on its output in the way that was discussed earlier for power amps?
 
I think someone like Arkless would be the best person to comment on what is the normal design practice for preamps.
As far as I know nearly all preamps have capacitor coupling on their INPUTS to eliminate any DC component from being amplified.
The preamplifer output may or may not have a coupling capacitor. If the designer of a preamp / power amp combination has designed the power amp with an AC input (capacitor in line to remove any DC input) then he will probably not have used a de-coupling capacitor on the pre-amp output as he knows any DC signal will be removed at the power amp input.
 
cpg said:
I think someone like Arkless would be the best person to comment on what is the normal design practice for preamps.
As far as I know nearly all preamps have capacitor coupling on their INPUTS to eliminate any DC component from being amplified.
The preamplifer output may or may not have a coupling capacitor. If the designer of a preamp / power amp combination has designed the power amp with an AC input (capacitor in line to remove any DC input) then he will probably not have used a de-coupling capacitor on the pre-amp output as he knows any DC signal will be removed at the power amp input.
Click to expand...

Yep they can be either. Many tend to err on the side of caution though....
A pre amp (or any amp) can potentially amplify any DC at its input but can also potentially give a DC offset which is not a fault condition. The typical 1-20mV or so present at the output of most power amps is a common example of this and a DC coupled capacitor-less pre amp can do the same.
As I said many manufacturers err on the side of caution and fit input capacitors or output capacitors because for one thing they can'y control what people will use with the unit and for another it's likely to result in less returns and fewer perturbed customers...

There are several factors that effect the suitability of an amplifier to be used actually down to DC:

Does the offset drift with time?

Does if drift with temperature?

Does the input offset current mean that the offset changes with the resistance the amp is driven from?

The later one is why the using a Naim amp without input cap, as you mentioned up thread, is a dodgy idea... not likely to be disastrous but could end up with say 400mV at output depending on precisely what is driving it...

Naim pre amps are a suitable example of a single rail and single ended topology (pretty common) which means that the outputs sit at half supply voltage (12V here) and so MUST have output capacitors.
 
The danger with using a valve amp with no load is mainly when it's a tetrode or pentode amp with zero feedback...
The output transformer primary can then act as a choke and very high voltages can be generated (thousands) which can cause flash-over internally in the transformer and damage it (possible flash-over in output valves as well).
It's unlikely with most hi fi amps as plenty of negative feedback is usually used and tetrodes or pentodes are not likely to be used "raw" but at least ultralinear, but it's wise to not use a valve amp without load anyway... just in case...
 
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