300W rms amplifier with only 2 output transistors possible?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi folks,

I have a new question. Last week I repaired an amplifier that use on the output stage only two TIP35 NPN transistors in a quasi-complementary output configuration. One of the channels failed at heavy load of 4 ohms going into short circuit. The manufacturer claims that the amplifier is safe enough to run at 4 ohms loads (two 8 ohms speakers in parallel) without issues at 200W RMS (400 RMS total power) using only two output transistors.

On most amplifiers the use of transistors in parallel is the most used configuration to avoid a sudden short circuit on the transistors junction at high output currents and very low loads.

The power supplies +50 / -50 volts rectified, maybee enough power to build a 200W Rms amplifier per channel at 4 ohms.

My question is if using only two TIP35 is enough for the operation at maximum power with 4 ohms loads. Datasheet claims that the TIP35 can run at 25 amp max collector current without issues.

And what about the TIP3055C with 15 amps max collector current should also be enough to work properly???, but the experience is totally different because I replaced the TIP35 with TIP3055 and failed again.

Today modern configurations use NPN-PNP output stages because both devices are cheap and price is the same, maybe this is the way to build today an amplifier. The former quasi-complementary output configuration came due the high price of PNP power transistors in the past.

The PCB has the copper traces to add 2 additional transistors (emitter and base resistors), strange the design from my point of view the manufacturer reduced costs at expense of durability and power safety using only two transistors.
 
Last edited:
NO WAY.
Even less with TIP35 or TIP 36: they have a large collector current capability (25A) but a very weak SOA dissipation, so they have big trouble with reactive loads such as speakers.
And with +/- 50V rails (100 V end to end) the Tip35C is barely able to cope.
maybe those transistors are not originals but a failed repair attempt.
What is the amp brand and model?
Any picture?
 
The amp model is an Altech XP-4000 I found only the picture of the XP-6000 model same as the XP-4000 inside. On the picture the Altech XP-6000 seems to use 4 transistors a bit better, but the power supply using only 4700uf caps is very weak. Transformer seems to we very good and enough to supply 400W rms total power.

This is the situation in Argentina, we think that the local industry and manufacturing can compete in quality and price with first brands such Crown, QSC, Alesis... but the user experience is totally different.

amplificador-2400w-altech-xp-6000t-crossover-oferta-D_NQ_NP_451011-MLA20471021980_112015-F.webp
 
NO WAY.
Even less with TIP35 or TIP 36: they have a large collector current capability (25A) but a very weak SOA dissipation, so they have big trouble with reactive loads such as speakers.
And with +/- 50V rails (100 V end to end) the Tip35C is barely able to cope.
maybe those transistors are not originals but a failed repair attempt.
What is the amp brand and model?
Any picture?

Customer complained today that the unit didn´t last two performances and failed again going into short circuit and smoke. I explained him that the design on Altech power amplifiers manufactured in Argentina is very week using only two power transistor on each channel. Maybee adding two transistors more on each channel will run safe without issues. Anyway at the end what remains here is the user experience. Thanks for the reply.
 
Last edited:
Hi folks,

I have a new question. Last week I repaired an amplifier that use on the output stage only two TIP35 NPN transistors in a quasi-complementary output configuration. One of the channels failed at heavy load of 4 ohms going into short circuit. The manufacturer claims that the amplifier is safe enough to run at 4 ohms loads (two 8 ohms speakers in parallel) without issues at 200W RMS (400 RMS total power) using only two output transistors.
You should have enough sense to look for corroboration for outrageous claims like that.
On most amplifiers the use of transistors in parallel is the most used configuration to avoid a sudden short circuit on the transistors junction at high output currents and very low loads.

The power supplies +50 / -50 volts rectified, maybee enough power to build a 200W Rms amplifier per channel at 4 ohms.

My question is if using only two TIP35 is enough for the operation at maximum power with 4 ohms loads. Datasheet claims that the TIP35 can run at 25 amp max collector current without issues.
It is not about max current capability when Tj=Tc=25°C
It is about the SOA capability when the Tj & Tc are far above 25°C
And what about the TIP3055C with 15 amps max collector current should also be enough to work properly???, but the experience is totally different because I replaced the TIP35 with TIP3055 and failed again.

Today modern configurations use NPN-PNP output stages because both devices are cheap and price is the same, maybe this is the way to build today an amplifier. The former quasi-complementary output configuration came due the high price of PNP power transistors in the past.

The PCB has the copper traces to add 2 additional transistors (emitter and base resistors), strange the design from my point of view the manufacturer reduced costs at expense of durability and power safety using only two transistors.
learn HOW to assess a design before you go accepting stupidly unrealistic claims.

Here's my method for assessing output stage capability.

For a BJT output stage, the maximum power output is approximately (sum of the Pmax of all the output devices) divided by 5
A single pair of 100W devices has a total Pmax of 200W. That gives a reliable maximum output of 40W
 
TIP35C

Obviously whomever designed these amplifiers has no clue as to what Safe Operating Area means

With +/-50v into a resistive load you require 1.9 = 2 TIP35C in parallel with this rail voltage to be safe with a 4 ohm load.

Now add the reactance of the speakers and the phase issues and a whole new ball game is required.

A typical woofer with it's approximate 60 degree phase difference between voltage and current will force an amplifier to dissipate twice the amount of heat as compared to a resistive load.

Now add in impedance drops due to reactance and current increases yet again.

Take into account the temperature derating of these TIP35s (62% of rated current ant all Vceo values and the amount of required devices goes up.

many years ago when we used these TIP devices we ran a +/48v regulated rail with 4 up and 4 down and we had a 3 stage power supply (Class G) which helps the SOA drastically.

Zed Audio Los Angeles CA
 
How do you figure that? Look at the SOA curves on the data sheet and think about the voltage across the devices at various power levels and loads. 50V supplies and 4 ohm loads? Think about a point where the output is at 25 volts. That's 6.25 amps into 4 ohms and that's right at the limits, depending on which device is used. Nor does it account for any reactance in the load. Or high line conditions. Also think about the opposite device which is seeing a high voltage and may still have to conduct current due to reactance. Never operate around maximum limits if you want reliability. (Moer beat me to it, and better!)
 
TIP35C

Conrad,

Most people have no idea how to calculate the number of devices required for a minimum load impedance.

Many years ago I made myself a bunch of spread sheets with all the power devices we used (and could use) in our designs.

I never bothered with 4 ohm minimum loads but only dealt with 2 and 1 ohm loads.

Stacking devices in series helps a lot as you do not typically go near Isb parts of the curve.

I did a 900w/ch amplifier with stacked TIP35C/TIP36C and 3 stage class G.

Rail was +/-80v regulated. The TIPs only saw +/-40v and I used 3 up and 3 down (The series stack was of course 6up and 6 down.

Built these for Hifonics 20+ years ago and to date I have never had a blown output stage.

I learned from my pro days, output devices are cheap relatively speaking, just add a few more up and down after CORRECT calculations have been done.

Today we only design class D as they simply do not have the issues that class B does. Two TO-220 devices on a +/-60v rail gets us 350w at 4 ohm and 550w at 2 ohm.
 
I had it running at clipping on 63V for a couple minutes, while attached to a 50lb chunk of aluminum, just to see if the devices would overheat. The devices were at 58C. I need to install a couple higher voltage caps before I wind it up on 92V rails and try again. This will eventually be running on 63V rails on a 4 ohm subwoofer.
 
Customer complained today that the unit didn´t last two performances and failed again going into short circuit and smoke. I explained him that the design on Altech power amplifiers manufactured in Argentina is very week using only two power transistor on each channel. Maybee adding two transistors more on each channel will run safe without issues. Anyway at the end what remains here is the user experience. Thanks for the reply.

When you did the repair you restored the amplifier to as designed condition. Unfortunately it is not capable of meeting the customers requirements. Adding more output transistors will most likely just expose the next weak link.

Your repair was correct. If you want to try and design an amplifier to meet your customer's needs that is really a different issue. Probably cheaper to buy a better unit.
 
Your repair was correct. If you want to try and design an amplifier to meet hour customer's needs that is really a different issue. Probably cheaper to buy a better unit.

Often mass produced amps are made down to a price.
Corners are cut on number of transistors and size of power supply caps and transformer.
I have opened up amps I have bought and been horrified by how little is in them. Tiny transformers that just could never provide the watts the amp needs.

I have a 100 watt Marshall amp. It has a 1 inch fan on it to cool 100 watt amp.
There isn't even a vent hole for the output from the fan, just an input hole.
Then the fans don't last 5 minutes. I had to replace mine because it was very noisy.
 
TIP35C

Temperature is the only "thing" that affects the SOA so I guess a higher idle current (bordering on low power class A) will heat up the devices quickly thus lowering the SOA curve.

I doubt that pro amps run at high idle current as it serves zero purpose.
 
To run safely with real 4 ohm speakers from +/-50V rails you need 312.5W of dissipation capability per output transistor. At 50 volts. Install two pairs of MLJ21194, and then hope the drivers are up to the task. Typical Jap TO-220 types usually will be, anything smaller may not be. If you can't fit the MJL variety, the MJW's would probably be ok as the only difference is the package.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.