Hi guys,
my first post so go easy
. I'm buying and modifying the gain of a O2 amp for a christmas present for my brother. His Soundblaster ZXR line out generates just shy of 2Vrms into 10kohm. On 1X gain my E17 drives his headphones without drama (Beyerdynamic Custom One Pro). So i've decided on a low gain of 1X for the O2. The High gain i'm not so sure of. I hear some headphones require 6Vrms so a minimum high gain of 3X (6Vrms/2Vrms) is my lower limit. I'm unsure about the upper limit of the input stage before any clipping artifacts. NwAvGuy: Op Amp Measurements, shows clipping starts about 6.8V with rails at 11.85V so say 6.9V at 12V rails. There is a voltage divider on the O2 gain stage input of 0.973, so 2Vrms in becomes 1.947Vrms. So gain just short of clipping is 6.9/1.947 = 3.54X. I guess 3.5X would be a maximum high gain to use? I ordered some resistors for 3.15X gain, should I get another pair for closer to 3.5X gain?
Neug
my first post so go easy
. I'm buying and modifying the gain of a O2 amp for a christmas present for my brother. His Soundblaster ZXR line out generates just shy of 2Vrms into 10kohm. On 1X gain my E17 drives his headphones without drama (Beyerdynamic Custom One Pro). So i've decided on a low gain of 1X for the O2. The High gain i'm not so sure of. I hear some headphones require 6Vrms so a minimum high gain of 3X (6Vrms/2Vrms) is my lower limit. I'm unsure about the upper limit of the input stage before any clipping artifacts. NwAvGuy: Op Amp Measurements, shows clipping starts about 6.8V with rails at 11.85V so say 6.9V at 12V rails. There is a voltage divider on the O2 gain stage input of 0.973, so 2Vrms in becomes 1.947Vrms. So gain just short of clipping is 6.9/1.947 = 3.54X. I guess 3.5X would be a maximum high gain to use? I ordered some resistors for 3.15X gain, should I get another pair for closer to 3.5X gain?Neug
Hi guys,
my first post so go easy. I'm buying and modifying the gain of a O2 amp for a christmas present for my brother. His Soundblaster ZXR line out generates just shy of 2Vrms into 10kohm. On 1X gain my E17 drives his headphones without drama (Beyerdynamic Custom One Pro). So i've decided on a low gain of 1X for the O2. The High gain i'm not so sure of. I hear some headphones require 6Vrms so a minimum high gain of 3X (6Vrms/2Vrms) is my lower limit. I'm unsure about the upper limit of the input stage before any clipping artifacts. NwAvGuy: Op Amp Measurements, shows clipping starts about 6.8V with rails at 11.85V so say 6.9V at 12V rails. There is a voltage divider on the O2 gain stage input of 0.973, so 2Vrms in becomes 1.947Vrms. So gain just short of clipping is 6.9/1.947 = 3.54X. I guess 3.5X would be a maximum high gain to use? I ordered some resistors for 3.15X gain, should I get another pair for closer to 3.5X gain?
Neug
my first post so go easy
. I'm buying and modifying the gain of a O2 amp for a christmas present for my brother. His Soundblaster ZXR line out generates just shy of 2Vrms into 10kohm. On 1X gain my E17 drives his headphones without drama (Beyerdynamic Custom One Pro). So i've decided on a low gain of 1X for the O2. The High gain i'm not so sure of. I hear some headphones require 6Vrms so a minimum high gain of 3X (6Vrms/2Vrms) is my lower limit. I'm unsure about the upper limit of the input stage before any clipping artifacts. NwAvGuy: Op Amp Measurements, shows clipping starts about 6.8V with rails at 11.85V so say 6.9V at 12V rails. There is a voltage divider on the O2 gain stage input of 0.973, so 2Vrms in becomes 1.947Vrms. So gain just short of clipping is 6.9/1.947 = 3.54X. I guess 3.5X would be a maximum high gain to use? I ordered some resistors for 3.15X gain, should I get another pair for closer to 3.5X gain?Neug
Hi guys,
my first post so go easy
Neug
You can sweat the technical details of gain but the best method is just to try it. You want a front end gain that allows reasonable rotation of the volume control.
From a technical perspective if you have 2 volts RMS available then that means that on battery power you have a max swing of around 5 Vrms (or less) available from the opamps. So front end gain no higher than around 2.5.
So 1k is the lowest you can go for the feedback resistor.
I guess ill get a variety of resistors then for the high gain setting, and try them out as needed. The amp will be used on AC. Right now he only has one pair of headphones, they work well with 1X. He wants to buy some high impedance headphone so I guess ill wait before refining the high gain setting for them.
Since its running on AC only is there any benefit in removing the batteries?
Cheers and Thanks, Neug
Last edited:
As above the source input plays a big part in the gain calculation. With a redbook 2v ish input 1x gain is perfect for sensitive 32ohm cans like the Grados. For high impedance cans like the 300ohm HD600/HD650 2.5x gain is perfect.
I believe the stock setup with 6.5x gain was to cater for low voltage sources, i.e. portable devices that are less than 1v.
I believe the stock setup with 6.5x gain was to cater for low voltage sources, i.e. portable devices that are less than 1v.
Batteries charged continuously 24X7, without proper charge/ discharge cycle will loose capacity. Batteries in storage will also loose charge (approx 1% per day for NiMh). Such batteries will loose almost all charge in 3 months. Low discharge batteries are exception(Sanyo claims 90% charge retention, test duration- 1yr). Other members may have different opinion on this.
BTW removing batteries helps run those voltage regulators a bit cooler.
IMHO for AC only operation - charge / discharge batteries to full capacity once every month to maintain its capacity & store them in safe place for any future use.
Or you can send those batteries to me
Would need more info and details but that doesn't sound right. Batteries of any type can't produce "hum". If you have no AC connected... where is that hum coming from ? What is generating it ?
Sorry, I don't think I describing this very well, I'm not talking about Mains hum.
When I use 2x 9v batteries (Only have these heavy duty Zinc Chloride) after a couple of minutes without fail the source signal I'm trying to amplify is replace with a very loud hum, kind of like a evil mix of leaking DC and oscillation (and this will continue even after powering off and then back on, and I have to wait for 30mins + before the amp will work again for another couple of minutes....then repeat .
This happen on both the O2 Amps iv built.
But, when I connect the DC power rails to my DC bench supply rather than batteries both amps seem to work without any problem (had them running for an hour on the bench supply with no issue at all).
A few basic things to check. Also the 9 volt PP3 type battery has very limited power density. Anything more than a few milliamps will soon pull the voltage down. Four dual opamps at ??? 5 ma each is 20ma. However
1) Monitor the battery voltage while the amp is on. Keep measuring across each battery. Is the voltage falling quickly ?
2) Measure the current drawn from each rail. This can be done without breaking into the circuit. Just place your meter across each diode D2 and D6 while on a reasonably high current range (say 200ma or 1amp F.S.D.). You need a high current range to minimise the volt drop caused by the meter. What current is being drawn from each rail ?
1) Monitor the battery voltage while the amp is on. Keep measuring across each battery. Is the voltage falling quickly ?
2) Measure the current drawn from each rail. This can be done without breaking into the circuit. Just place your meter across each diode D2 and D6 while on a reasonably high current range (say 200ma or 1amp F.S.D.). You need a high current range to minimise the volt drop caused by the meter. What current is being drawn from each rail ?
Yes, a fresh battery goes from 9.88v to 9.68 in 20 seconds
D2 = 14mA
D6 = 3.8mA
The pos and neg currents are a bit unbalanced... although that could be an artefact of the measurement procedure reading across the diodes.
No idea why it should start to hum on battery tbh. I think you need to monitor the battery voltages as you run it and see what they fall too when the fault appears.
Strange thought... bet this is the problem... check the voltages on the IC's too. Could there be something weird going on with the FET switches and battery cut off ? Yes Is the battery cut out circuit modulating the rails as the voltage approaches the cut off point. You might need to fiddle the cut off value if only using 9 volt batteries by altering R5 or R9.
I think its all down to the characteristics of the batteries and that off load the voltage will suddenly rise again. Rechargeables don't behave quite the same.
No idea why it should start to hum on battery tbh. I think you need to monitor the battery voltages as you run it and see what they fall too when the fault appears.
Strange thought... bet this is the problem... check the voltages on the IC's too. Could there be something weird going on with the FET switches and battery cut off ? Yes
Is the battery cut out circuit modulating the rails as the voltage approaches the cut off point. You might need to fiddle the cut off value if only using 9 volt batteries by altering R5 or R9. I think its all down to the characteristics of the batteries and that off load the voltage will suddenly rise again. Rechargeables don't behave quite the same.
The pos and neg currents are a bit unbalanced... although that could be an artefact of the measurement procedure reading across the diodes.
No idea why it should start to hum on battery tbh. I think you need to monitor the battery voltages as you run it and see what they fall too when the fault appears.
Strange thought... bet this is the problem... check the voltages on the IC's too. Could there be something weird going on with the FET switches and battery cut off ? Yes
I think its all down to the characteristics of the batteries and that off load the voltage will suddenly rise again. Rechargeables don't behave quite the same.
Think I'm going to get a couple of rechargeable batteries and try that before I spend any more time debugging (or will end up stabbing it with my soldering iron!
) Here is a audio clip of the fault when using batteries
https://soundcloud.com/peps1-1/o2-amp
(And, thank you for your help with this!)
Yes, that sounds like it could be the rails doing something odd as the voltage falls. I don't know if you've built it on the proper PCB and/or used all the correct parts etc but I'm certain now that that is the problem. I would also want to be sure on that unequal current draw from each rail. You could temporarily remove and replace D2 and D6 with 10 ohms and measure the volt drop across each. The current draw should be equal on each rail. If it is not then you need to find out why.
What you do about it depends on your intended use of it. If your using dry batteries then you don't need the power management stuff at all.
What you do about it depends on your intended use of it. If your using dry batteries then you don't need the power management stuff at all.
You would need to be sure that LiFePO4 chemistry supports trickle charging (as in the O2) Li-On does not. The O2 will charge batteries as long as there is a differential between Vin and Vbattery. Again you need to know the characteristics of LiFePO4 and what voltage (current limited) is needed to bring them to full charge. How that voltage rises with charge determines how long they would take to recharge.
They are all shown as 0.22uf decoupling caps. It will all still work if one or more were missing.
One thing I noticed and it may be correct but check
The PCB in your picture clearly shows a + (plus) symbol on each battery location. As it stands, the + terminal is the one that will fit the - terminal on the battery. They look reversed. Check it !
One thing I noticed and it may be correct but check
The PCB in your picture clearly shows a + (plus) symbol on each battery location. As it stands, the + terminal is the one that will fit the - terminal on the battery. They look reversed. Check it !
They are all shown as 0.22uf decoupling caps. It will all still work if one or more were missing.
One thing I noticed and it may be correct but check
The PCB in your picture clearly shows a + (plus) symbol on each battery location. As it stands, the + terminal is the one that will fit the - terminal on the battery. They look reversed. Check it !
You are spot on the battery clips are reversed, easy to do
cheers
FRED